> No checks are done with HMRC for their income levels either.

I'd have to look it up, but some level of background checks are explicitly banned by the RIP act. It was put in place to allow local councils to "snoop" but only in defined situation. (for example if you have noise complaint, the council can't make a recording, but they can accept a recording you make.) but on the other hand they sometimes can inspect your bins to make sure you are not mixing waste streams.

> Of course, if a future government just wants to round up dissidents and send them to camps,

The UK constitution is pretty vulnerable to this. However, even a strong written constitution isn't going to stop that if it becomes socially acceptable.