Link to how Google Wallet works:
https://blog.google/products/google-pay/device-tokens-google-wallet/
As can be seen, the security process is far more watertight than using a traditional card.
As already mentioned, multiple cards can be easily stored.
As an aside AFAIK the DVLA are working on a way of digitising the driving licence, which would then be compatible, and thus a legitimate form of digital ID.
It's also worth mentioning that the feature where you have to have your phone 'open' to make payment is not optional. Unless you set up a standard unlock process on your phone the App simply will not function in the first place.
Here is the UK listing for banks that support Google Wallet:
https://support.google.com/wallet/answer/12059326?hl=en-GB&co=GENIE.CountryCode=GB