I think making some little jokes sometimes lighten ups this forum
Now, addressing your concerns. I think you are just getting worried for no reason. I bank with lloyds, and they have an even stricter online checks. On my online shopping, when I have filled all card detials, I am transferred further to lloyds website where I enter a full username and a full password attached to this username. This is to prevent fraudulent use, eg if I loose the card, or someone somehow gets my card number, expiry dates etc, and then uses the card for shopping online.
But never ever I had problems when I used my card for government agencies. Banks know that no one will go to shop for things with UKBA. UKBA does not process your card in the same way as you would do for online shopping.
And rather than worrying speculatively, why not just relax. There won't be any problems, but even if there was a payment problem, it won't be the end of the world, will it. They will ask you for alternative method of payment. If you had such grave concerns about your natwest card, then you should have sent them a cheque. Would have only added a 3-5 to the process. Hardly life changing I guess.