[mrbarlow]

Orange does have a "Call Abroad" pre-pay SIM and some of the rates are quite good, but there seems to be a limited number of countries mentioned. Whether these are the only countries you can call or are the only ones with a "special" rate, I don't know. It might be worth considering if the country (or countries) you're planning to call are on the list.

I couldn't tell you what the other pre-paid SIMs are like for overseas calls but the UK being what it is I doubt any of them are particularly cheap! By all means check out all the operators (and MVNOs) but you're probably better off using something like Jajah to make calls.

Edit:
Have just looked at Mobile World (MVNO owned by Carphonewarehouse) and that seems to have some reasonable rates for overseas calls...