I just got back from England and the Tesco phone worked very well,
EXCEPT that you can only register for top up etc with a UK address.
Even though I do have a UK credit card and a UK bank account, the
statements are sent to my USA address, so I could not register them.
The only way that I can top up the phone is to go into a Tesco store
and buy a top up voucher (or other locastions - didn't check that out).
The best option seemed to be to buy the phone, wich comes with a sim
for regular pricing, and on my GBP39 phone I got GBP10 of free calls.
Don't use that sim, but buy the Texco EXTRA sim for under GBP2. Put
this in the phone, allocate the GBP10 to that number, be sure to top up
with at least GBP15 to get the EXTRA rate, and try not to make too many
calls before the rate kicks in - around 48 hours.
Apart from the minor inconveniences it worked pretty well.
Thanks to everyone who helped me to come to a decision.