I'm unsure if there are any civil S511 installations left in the UK. I know that Bristol, Cardiff, East Midlands, Newcastle and Southend once had them but I believe those have all gone. As for its Plessey contemporary, the Watchman, it lives on at Leeds Bradford and perhaps still at Humberside, Teesside and Southampton. Maybe the MOD airfields still have the Watchman too.
By the way, did the S511 use magnetron technology rather than travelling wave tube ?