An interesting question - I think history suggests the Britain would have been involved. I think it was the area of land that Belgium occupies rather than Belgium itself that was important... recognition of why so many wars seem to pivot about that part of Europe.

Historically the Britain had almost always seen the neutrality of that area as a key part of the balance of power in Europe which protected Britain - both through different treaties and through providing funds to the Dutch to mantain the fortresses there.