SWBKCB
that's an easy answer. The metal is at LHR and LGW. Most of the local catchment for BOH and SOU simply makes the road or rail trip to the London airports for almost all their air travel requirements. London is not so far away.
The expansion locally to BOH/SOU will only happen when it makes sense for the airlines. In other words when The London airports a too full or too expensive.
My previous point was explaining how KLM approach things. The other legacy carriers have different philosophies for different reasons and in my view will never be seen at most UK regional airports.
The LCC are different again, they simply follow the money irrespective of their home base, they have no hub to protect. Expect to see steady LCC growth continue.
FF