Over the last few years I have noticed that Vodafone UK don't put SIM locks on their contract phones. They have you under contract so I guess it does not concern them if you choose to use the handset or device with another network as they get their money anyway.

How many other networks do this?

I have found it very useful when I take my handset abroad as I can use a local SIM on PAYG without any hassle.

Really, I have used ALL networks and I find they all lock phones, even Vodafone.

No SIM lock on my last three Vodfafone contract handsets. I asked about it and it's been their policy for some years not to lock them for the reasons stated above. They still lock PAYG handsets though!

Ah ok, interesting

My last contract handset on o2 (Samsung Pixon) was unlocked.

If you get your contract via Carphone Warehouse your phone should be unlocked on any of the networks as they use the same stock no matter what operator you choose.

O2 is the only other company that does it.

Phones 4u + carphone warehouse will also give you an unlocked phone no matter which network you get it on.