I think mobile payment as a sector has a lot of potential, but there are going to be a few big winners. The really big win for a domainer would be thinking up a brand that could be offered to a big player, and I have a few domains attempting to do this.
Your generics are good, but I feel the mobilepayments.org.uk is going to be limited to an informational site which you can develop to track the sector and advertise services. mobilemoneytransfers.co.uk I could possibly see a sale but big companies will prefer brands. If there start to be affiliate programmes for mobile payments, your .co.uk becomes a lot more attractive.
Rgds