Well, to close out my original inquiry (enquriy).

I arrived in Terminal Four and found no shops that sold SIM card starter kits or prepay packs. Boots, HM Post Office, WHS only sold top-ups and told me that they thought that on the air / departures side (where I could not go, just arrived) I could buy a starter pack. Purchased a starter pack in town at T-Mobile and it worked fine. Must say that I am very impressed that you can go to the post office and top up with any number of companies - the U.S. carriers could learn from that.

Thanks to all for all the advice and information!