if you google search the words "free uk sim " you will get many options from different service providers, i would suggest also that you check the pay as you go tarrifs as you know some providers are more expensive to pass calls though their network than others.
Bill, www.freesimcard.org.uk is a good place to start looking. I guess you'll need to get it posted to a UK address though. Maybe a relative could order one & have it ready for you on touchdown?
SANTY....am I ok to put that web address in the post?