Sim card doesnt work

It means you have a iPhone locked to a carrier.

FYI: Only that carrier can release the lock officially, not Apple or anyone.

Bring it back to the seller for a refund.

I know, I just told you it's locked to a carrier.

You need to talk to the carrier in UK.

There's nothing we can do for you here.

Yes and are you in the U.K?

It's probably locked to a U.K. carrier.