@Draug ... yeah, it's a hill, but some people are out of breath when they walk up the stairs. Average people don't do it.
@SYnapse ... if the climb is 9+ hours (which I imagine it is, but I'm guessing), you're going to want two packed meals and plenty of small packaged food, specifically food that you don't mind eating frozen (because everything will be frozen). You should also pack plenty of sugars and carbs, especially if it's foggy, because you won't have any views to keep your energy up. Once you're above tree line, you probably won't be able to see the step in front of you.
I just looked at the peak on Google Maps and it looks like a pretty steep hillclimb. If there's thick snow, you're going to need a 3500-pack at least, because you're going to want to pack a lot of layers. If it were me, I'd wear something as a base underneath out of the house/hotel/etc in the morning and throw on a fleece on top as a second layer. You're going to want to pack your third and fourth layer, and potentially a fifth layer (if you do, make it a puffy jacket). For your third, another thickish fleece is fine, and for a fourth, you'll want a windbreaker of some kind. That'll essentially cancel out the windchill on your torso. The fifth layer wouldn't be useful until you're above tree line, so if you don't go that high, don't worry about packing one. If you do, it's a good precaution to take.
No matter what mountain you are climbing, you want to bring a headlamp (battery is fine) as well as a spare something-or-other, be it a household flashlight or another headlamp. Obviously you don't want to bring anything that uses oil, because it'll freeze and you'll be screwed if it gets dark. In an ideal world, you won't need it, but if you do manage to stray off the path or get trapped (don't let your ego get in the way here), it's the one thing that'll keep you alive in an overnight storm.
If you aren't going with a guide, go with someone that's got a bit of climbing experience. From what I'm reading now, going off the path at higher elevation leads to some pretty dangerous terrain (particularly a cliff face, which, even if you don't walk off the side, will give you the biggest heart attack you'll ever have). Bring winter crampons, not regular mud crampons - 12 spike would be best - and if you don't have some, rent some; they aren't worth the price for one trip. Same goes for mountaineering boots - best to rent unless you become a regular climber. Check the sole on your boots before you wear them out, because a flexible sole isn't sturdy enough for winter use; you want it to be quite stiff. Most people add one to their shoe size (e.g. I wear 12.5 men's shoes and I wear 13 boots loosened up a bit - check sizes in the UK; I don't know if they are different).
You will need an ice axe, and no, trekking poles or ski poles don't substitute. You can rent this as well, and I imagine there are good places within the park boundaries, but you'll find them cheaper if you get them further away. They'll help you out with the shaft height, but as a baseline, I'm 5'10" (70 inches, ~180 cm I think) and I carry a 55"/140 cm. You can also have an ice tool if you'd like, but it's not necessary, so if you can't get one free, don't.
If you don't have some of the gear, it's not like you are putting yourself at grave risk, but the boots, crampons, and ice axe are an absolute must no matter what. The company or group you go with might want you to have a helmet, but that's up to you. You would be well served to find a local hill (doesn't have to be tall, just something relatively steep) and test out your gear, particularly your crampons and boots, because it is really weird to walk in them for the first time. If you can't find time to do this, at least try to get a feel for them before you go on the climb, and learn how to get them on and off without sitting for too long (because it gets really cold really quickly when you sit). Learn to walk with your ice axe (adze forwards, pick backwards, always in your uphill hand - you'll see what that means then when others do it) and, if absolutely nothing else, learn to self arrest (https://www.youtube.com/watch?v=YyRF6AjAI94&feature=player_embedded). You probably won't need to do this, but if you do, you'll be glad you can do it the right way.
Know how to navigate, even if you aren't leading your group. A GPS tracker can freeze or get blocked by nearby cliffs and hills, and either way, it won't substitute for basic navigation skills. You probably won't have to worry about avalanches, but upon googling "Ben Nevis," one of the first news stories was about a climber injured in an avalanche last week, so be aware of the risk levels before you go, even if you aren't the group leader. A good page to check out - http://www.walkhighlands.co.uk/safety/avalanche-safety.shtml.
In regards to your pants, you don't necessarily have to wear something tight to your skin, but you should have a similar base layer to your top and snow pants on the bottom. If they are baggy, PACK DUCK TAPE. You will snag your crampons on your pants at some point, and they will tear. Everyone does this, no matter how experienced. If you happen to tear a big enough gash, it can be dangerous to you or other people walking near you.
In general, always stay with the group, and never leave anyone alone. If you plan on going alone, get rid of that plan right now and find someone to go along with, preferably someone that's better than you. Don't outrun your group, and if they are going too fast for you, don't be afraid to speak up.
You might be able to get away with exposed skin on your face, assuming the temperatures don't plunge, but I'd be prepared with a facemask and balaclava just in case. Wear a warm winter hat (you've probably got one of those already) and have ski goggles in your pack, again, just in case. You might benefit from having gaiters for your ankles too.
I know all of this sounds like overkill, but if you're resourceful, you can borrow or rent a lot of this stuff. Any regular climber knows you'd rather have too much than too little.
One practice that I started doing after the Great Mt. Thor Fall of 2012 (painful memories) is leaving a note with a concierge if you're staying in a hotel or some other person that knows the group. List the time you are departing, the amount of time you expect to spend on the mountain, and the route you're taking up. If you don't leave it with the hotel, also list where you are staying. If you do this, ****be sure to tell them when you arrive****. If not, you'll probably be paying for a rescue attempt, and you probably won't be climbing in the UK again.
I just listed a ton of stuff, most of which you probably already know if you're well prepared (or your group leader knows, same idea). Much of it will end up proving irrelevant, and hopefully you won't need to heed any safety advice at all, but mountains deserve your respect, even if they aren't all that tall. There is significant danger on each one of them, particularly in winter, and you have to be ready to go. I like to compare it to texting and driving - you'll get away with it 7,374,000,000 times, but it only takes once. Some mountain someday is going to be having a bad day and decide to send an avalanche your way, or it'll decide to pull a prank on you and hide the edge of the ridge behind a cloud of fog, and you would be wise to be prepared if one of these things were to happen. You're also highly unlikely to get something like frostbite, but I've had it before, and it's definitely something you want to make all efforts to avoid.
Otherwise, enjoy it, and enjoy every aspect of it, not just the summit, because if it's foggy and cold, the summit won't be much. Enjoy the scenery when you can, take in the snow-covered trees, walk next to the frozen-over river and hear the water moving underneath, take in the air, or just enjoy the company of your companions, whatever you've gotta do. If you don't enjoy it, don't do it again, or only do it in the warmth or whatever. Just take all precautions and warnings seriously.
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