a great solution
If you have ever used Felica in Japan or Octopus in Hong Kong you will appreciate the benefits. In HK especially you can use it for payment virtually anywhere and many buildings use it for ground floor door access also.
I admit, I feel nervous about having my octopus automatically set to reload from my bank account so I don't do that, but you can set a maximum amount per transaction and a maximum amount per day on the card to provide some safeguards.
there probably is a minimum transaction level but I haven't found that yet, everything I have wanted to purchase has worked.
But I am sure the UK mobile operators can bugger this up and provide a more expensive, less capable system.