Just had the same issue myself with buying a new PAYG phone.
I would look at web site "e2save" they are part of Carphone Warehouse and 99% of the PAYG phones are unlocked.
What I did was find out which network is selling the phone you want at the lowest price, then once you have used your £10 Airtime on the new sim you can then put your current sim in the phone.
I got a Samsung Tocco Lite for £68 + £10 airtime + 4Gb Sd card on vodaphone for £78, I have now used my £10 airtime up so have put my current orange sim card in the phone.
If I had bought the same phone on orange PAYG it would of cost me another £12 and because I bought 2 (one for me & one for the wife) it saved me £24.