Yesterday I bought a PAYG SE K800i from my local CWP store and was
amazed to discover that it was unlocked.

Is this common?

I thought all network-subsidised phones were locked to the subsidising
network and did not accept another network's SIM. Not so in this case
apparently. 02/Voda/Virgin and T-Mobile SIM work fine.

The phone cost £180 PLUS £20 airtime credit = £200 in all, which seems
about thirty quid cheaper than a SIM-FREE K800i purchased online.