What you have to remember is that NATS has two halves; NATS En Route Limited (NERL) (ATCCs) and NATS Services Limited (NSL)(Airports). SSR interrogators are generally (but not always) 'owned' by NERL and the information is 'bought' by airport operators, even those where ATS is provided by NSL. Although some RAF airfields have their own interrogator, some 'buy' their SSR from NERL too.
All airports have to be treated equally, hence the cost to the airport, whether or not NATS(NSL) provide ATS, is the same.
So the 'government' has created a system where it is in many cases cheaper to install your own SSR interrogator than to buy into the existing infrastructure. This can only be a bad thing for the total cost and the amount of RF pollution. I wonder if this is part of the reason that RF pollution is alleged to be so much worse in the UK than the US (a deduction from one of the key needs to rapidly move to mode S).
There seems to be a fundamental regulatory/cultural problem with organising how to provide a reasonably cost/safety balanced service to the GA community. Having watched various controllers in action and visioning a bit into the Mode S world - It seems what the average GA aircraft would like is a radar FIS where the FIS provider is prompted using something akin to the short term conflict alert and the airspace intrusion predictor to provide information. Using the mode S data you can even imagine a tele-prompter type script (i.e. G-BBBB - twin traffic 2 o'clock 3 miles crossing left to right 300 feet below, unverified, or on your course you will enter CAS in three miles)
There was a company (Stormscope I think) who offered a low cost SSR receiver several years ago.
There are a range of these, but you are probably talking about the Ryan 9900B (or possibly one of the range and altitude only ones for about $1k)