You can always opt to use a different payment method during the checkout than the one you have stored. There have been a few instances in the past where I opted to pay directly with my debit card rather than my stored credit card so I could avoid adding a charge to the credit card. It would be nice to store another one, but there is a way to pay with a different method already.

Ofcom are the ones who require a credit card.

1) Age assurance methods – which include age verification, age estimation or a combination of both – must be ‘highly effective’ at correctly determining whether a particular user is a child.

2) They include: open banking, photo ID matching, facial age estimation, mobile network operator age checks, credit card checks, digital identity services and email-based age estimation;

In the UK you have to be over 18 to obtain a credit card.
Since Steam can accept credit cards that is the easiest method for Steam to abide by the new UK laws for age verification.

Valve has no right or authority to have access to a users government ID. And that each government has a different way to design and handle IDs, it is too much hassle for Valve to deal with to keep track of IDs from dozens of different countries.

Wait. You can't have more than one card saved to your account? Really?
Well just from a practical perspective that needs to be changed.

The AGE verification card should be invisible and unusable. There is a simple "remove" button and that's it.

Originally posted by Pocahawtness:

The AGE verification card should be invisible and unusable. There is a simple "remove" button and that's it.

Quick question Why?
Why would you want to be unaware of what card is linked to your own account?
And what does it matter if its usable or unusable. Whether you use it or not is up to you. If you don't wanna use it.. don't use it.. Why do you need Valve to restrict you?