How to tell if my iphone 4s is blocked in the UK?

what do you mean by blocked? Can you give us more details in what you're trying to do.

Chances are yes ,The only source of non carrier locked iPhones in UK is Apple retail stores or Apple.com/UK

All others are carrier locked ,no exception ,unless the carrier has formally unlocked it

You could call AppleCare in UK and they may tell you the status

Or do you mean a carrier has blocked the iPhone IMEI on UK networks ???? as a result of theft etc