please forgive my ignorance
but I have to wonder, how will you top up your account?
with the pre-paid service, you buy 10 quid card, you top up your phone with 10 quids and then you get to use those 10 quids. The problem is, the telco doesn't get the full 10 quids, some amount goes to the seller that sold you the card. Plus those 10 quids actually include the VAT* amount for the card. So in reality, the telco _might_ actually get 5 quids out of the 10 quids you paid for the card!
Now when you use the NFC payment system, if you have 10 quids in the phone, then you expect to be able to _pay_ for an item which value is 10 quids! The telco will have to honer the payment of 10 quids.... but to my understanding (if the above example is correct), they never received 10 quids from you!
so how does it work?
* I don't know about other countries, but in country I am in right now, calls and SMSs are VATable, and all pre-paid top-up cards are VAT inclusive.