Alright, so I exaggerated a bit about the redundancy of lines. But the UK being the UK is quite relative if you know that EU law is above UK law.
But anyway, if you make it one country, people will hop to France to avoid taxes (though if you want to talk tax avoidance, look up the Dutch sandwich tax evasion. Those few people moving across some line are peanuts).
I agree, economies of scale are good. But our competitors are not France, Germany, UK. In fact, they are our close allies against mainly Asian (Chinese) producers, transporters and service-providers. You may feel like Dutch now, but I live in Asia now and I can tell you that the difference between Europeans is mainly in the the of accent they speak English in. Cultures are incredibly alike, which I think is a good thing.
Economies of scale, great! Think big. So skip low countries, unite EU and be part of the G3.