What App do I need to use NFC on an android phone to pay for things in the UK? I'd like to know if it's possible to use NFC on an HTC One X to pay for things in the UK and if so, what app do I use?

Google wallet doesn't seem to be supported here in the UK.
A more generic answer, not specific to the UK alone:

As Ross pointed out, there is not "one specific app to rule them all". That payment stuff is very much vendor-specific – whith "vendor" meaning the company dealing with the payments. Unless several shops/companies join up for a "standard", it might very well be you'd need a separate app for each shop even.

So this question cannot be answered by a "this-is-it" app, but rather would become a pretty long list (Ross' answer already indicates such), which would be subject to (almost daily) changes whenever a new "vendor" joins or another gives up.

To give you an example: Here in Germany I know of two grocery chains using NFC payments, each using its own app – though both belong to the same "mother corporation". If not even those can figure a "joint approach", I very much doubt there will be a "global approach" any time soon.