We bought Virgin pay as you go phones in the US which we use whilst over there. About $30 each from Target if my memory serves me well.

You can top them up on line from the UK to keep them active and you only have to top up every 90 days with a minimum of $20.

We visit regularly though so this may be more worthwhile for us as opposed to someone who only visits infrequently - not sure which category you might fit into.

Dianna