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.
It's never going to change.
In the UK you can file an IFR FP from one Class G airport to another Class G airport, at 2000ft, but it's pointless because it doesn't give you anything in the way of service. You may as well file VFR and drill through any IMC you come across.
A while ago I did some notes on this
here.