So, regardless of who will qualify as first between England and Belgium, the second half of the round of 16 will have 7 european teams out of 8, Colombia is the intruder. It's already certain that an european team will be in the semifinal, and if Colombia loses against England or Belgium, we'll already know from the round of 16 that there will be an european team in the final (even though statistically that's not hard to predict before any World Cup).
England and Belgium should think if Colombia is a "safe" enough adversary worthy of a tactical defeat to avoid the half of Brazil, France, Argentina and Portugal... but as I said before, careful what you wish, you just may get it!