In the long run, Britain will always tend to ally against the dominant continental power. This is because Britain's enduring interest is to ensure that western Europe doesn't unite under one power, as this would threaten Britain itself. Before German unification, the dominant continental power was usually France.

(I believe this is also why Britain joined the EU in the first place - it's not a threat if Britain has a seat at the table.)

The upshot of this is that Britain was always going to enter WWI once it looked bad for France - if not Belgium, some reason would have been found.