For payment, all the networks will be offering “stored value accounts”. You load these us with bank account/debit card/in a store, and it’s like having cash in your wallet. When the money loaded up is spent, you have to load it again. So if you have £50 in your SVA, that’s the most you can lose from it.
In addition, you’ll be able to add credit/debit cards to the wallet on your NFC phone, and use them at the same places you can use contactless credit/debit cards. More and more places accept these (120K terminals out there at the moment) but retailers are somewhat reticent to upgrade their POS again – they did it fairly recently for chip and PIN. Macdonalds is probably the biggest retailer with them but it’s coming to Asda soonish.
But that’s just payment – NFC/mobile wallets can do a lot more. Oyster cards, event/travel tickets, loyalty cards, vouchers, gym passes, door entry… all can be put in a mobile wallet. It’s probably going to be more useful for these than for payment.