The number of active credit card holders in the UK is somewhere in the 35 million range. The number of adults is somewhere over 56 million. There is quite a mismatch there. Unlike the US, owning a credit card is not standard for most people in the world.
People tend to get them for online shopping abroad, and for travels out of their currency zone as a back up.
It's credit card OR id though. People like Experian are already offering age verification systems that check passports, driving licences etc. The government also have their own GOV.UK Verify programme which has multiple providers which prove identity and thus age.
If you can login to your government tax account, or apply for a DBS, you'll be able to prove your identity.
I don’t know if a debit card counts, because they are 16+. Many of us took a while to move onto credit cards but still had access to cashless, online and overseas payment.
People tend to get them for online shopping abroad, and for travels out of their currency zone as a back up.