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.
There was a company (Stormscope I think) who offered a low cost SSR receiver several years ago; what happened to it I don't know but it was NOT the Mode S only receiver offered by the SBS-1, it could receive mode A and C too.