Surprisingly small contract. It's interesting to see that a full government contract for a payment provider is a fraction of a US mid-size company's cloud bill. I am constantly surprised by things like this. Here's another: there are more foreigners in Taiwan (total pop. 25 m) than in China (total pop. 1.4 b).
Interchange average in the UK was about 0.2% last time I looked (which was a few years ago), it't not much anyway. Also the government doesn't really do card transactions. I imagine this is for fairly rare things like renewing your passport, booking a driving test or buying a title copy. Oh and visa fees maybe? Small beer anyway, it's not like people are paying taxes via card.
But regardless this contract is _not_ for HMRC payments, its for gov.uk pay which is basically a centralised service that other services can use.