It's a huge discussion topic but basically the southern UK has a near-watertight division between the IFR controllers (who handle only traffic flying on Eurocontrol flight plans, nearly always in CAS) and other "GA" services.
It's tied into ATC funding, etc, etc.
Not clever use of airspace and resource to me. Nobody else does it this way. The airspace above isn't always full of IFR traffic, and the airspace below is sometimes unsuitable - crowded at choke-points with plenty of infringers.