It sounds like I should clarify a bit in terms of both England and Russia (I'm also happy to answer questions via PM, but remember that you'll get an answer based upon my reading of the board, not the press. The press has a lot more information than I do.)
It's ideal to play the game in a way where you have multiple good options and your opponent has only one good option, the one you wanted him to have. Generally, the players with multiple good, viable options are doing well and the ones with only one path forward are entirely dependent upon an ally.
For example, England in this position appears to be at war with France and at war with Germany. Thus, even though Russia is growing and is a long-term threat to him, England does not have the option of fighting Russia; its survival is dependent upon him. On the other hand, France currently can fight England and profit, can stop that war and fight Germany and profit, and possibly could choose to fight Italy and profit.
[I was going to lay out some future scenarios here, but don't want to unduly influence the game. So, let me speak more generally instead, in a way that I suspect will answer England's question as well.]
A decent rough way to think about flexibility is to count how many units are needed to defend what you've got, and the more units you have left over, the more flexibility you have. Even though at the moment you are working with Italy and Turkey, that won't last forever, so let's think about what's involved in minimally holding your gains down south:
A minimal defensive line with Budapest looks something like F Rum (BLA empty), A Gal S Rum, A Bud S Vie, A Vie S Bud. Four units, and if you're faced with armies all along the front, then you guess a bit with Gal and have something where at least a stab isn't guaranteed to get a center.
A minimal defensive line with Ser instead of Bud? You can't defend Ser: Bud - Rum, power 2 attack on Ser.
A minimal defensive line without either one? A Boh S Vie, A Gal S Vie, A Vie H, A Ukr S Rum, F Sev S Rum, A Rum, and BLA empty (if it's no longer empty, you have a more active defensive line, but it's active). Six units.
So, to summarize, with Ser - Bud, you set up a situation where you need six units to protect three centers (Vie, Rum, Sev), while if you take Bud and Turkey stays in Serbia, you need four units to protect four centers (Vie, Bud, Rum, Sev). In other words, Bud is worth three centers to you in terms of the extra flexibility! For three centers, this immediately becomes the largest problem you've got, and so whatever else you thought you wanted to do, you're going to be solving this one first.
[As a general rule, if you find yourself using more units than centers, it's an indication that you are in an unstable situation, and will shortly either be advancing or retreating (this rule of thumb changes somewhat in the endgame).]
Let's ask the same question for England last year. As England, imagine you could choose between two scenarios:
A) Pull everything back, disband because you lose Norway, and have three units to defend your three home centers.
B) Take Denmark, knowing that it takes two units to defend it (plus Russian help) if Germany keeps his second fleet, leaving two to defend your three home centers.
As before, using 2 units to defend 1 center is a liability, and if you're in a war for survival, it might be a fatal one. You're now tied to using as much of your power to defend Denmark as you are devoting to defending three home centers, and surely you don't value those two things anywhere close to equally.
I will encourage all of the players to do this calculation for themselves -- look at every power, count how many units they need to defend their current holdings, and count how many extras they have (in the case of a power under severe threat, this might be negative; if so, that's a good indication that the power is about to lose centers unless something drastic happens, and that this player should be willing to take major risks to change the situation). The more extras, the more influential their decision-making and thus the more time you should be spending focusing on...helping them make those choices.