I'm thinking of the more linguistically diverse parts of the world, and what might make Britain more like them.
One thing that's clear is that fudging the survival of various languages to the near-modern era doesn't necessarily achieve much. The ultimate barriers are nationalism and the development of centralized states, both of which have contributed to driving most European languages into, or towards, extinction.
Rule from without up to modern times would be very helpful, but that's challenging for more than, say, Ireland and the small isles. Otherwise, surviving English kingdoms (geographically English, that is) or some sort of pseudo-Swiss federal arrangement would probably be the best bets.
One thing that's clear is that fudging the survival of various languages to the near-modern era doesn't necessarily achieve much. The ultimate barriers are nationalism and the development of centralized states, both of which have contributed to driving most European languages into, or towards, extinction.
Rule from without up to modern times would be very helpful, but that's challenging for more than, say, Ireland and the small isles. Otherwise, surviving English kingdoms (geographically English, that is) or some sort of pseudo-Swiss federal arrangement would probably be the best bets.