They tend to be quite expensive. But a friend bought a "cheap copy" one in spain for about €60. He had quite alot of problems with it though so beware!
known as "dual sim" phones

I got my spare phone unlocked in Folkestone for £10 and now have one for the Spanish Sim card

Just google your phone model and ask for unlock codes. There are plenty free on the web.
Depending on how long you are in U.K., for it may be cheaper to just pay roaming charge rather than PAYG sim and top up.

Wait until you get to spain and buy a phone that's not locked to start with? It doesn't seem possible in the UK but it is here.

The one she wants is expensive unlocked, but free when it`s locked into an 18 month contract.
All you really need s a phone that rings and you can dial out from.

you cannot, the reason for this is that each network has a network number and the phone is locked to the network number not the operator. many times in carrefour i heard arguments between british people who had bought vodafone or orange sim cards thinking that orange was orange or vodafone was vodafone, but its not. vodafone in spaoin is airtel who operate under the name vodafone under license and their network number is different. orange is the old amena which was never very successful and overnight one day they became orange. that night all their shops changed from green to orange. !!!

spanish / movistar roaming charges are not unreasonable. I had a spanish movistar phone that i carried around europe late last year and i found in many countries it was cheaper to use that to call spain and uk than using a phone of a national operator of the country i was in.

Hi, yes ive got a dual sim phone, had it 2 years now, its great, mines a Samsung one, and I got it off ebay... had a CECT one to begin with and that was rubbish.. so I would recomment buying a decent brand..

edit to add... mines a Samsung D880.. and ive dropped ot SO many times... but it still works...lol