Credit and Debit card checking is not as easy as you might think these days.
They used to all come with the 16 digits raised from the rest of the card. That was to ensure that they could be used in the manual imprinters. When more modern technology was introduced they were still provided as backups but this is long discontinued.
Now most UK cards are flat, with the number appearing in various places on the card. A large number don't even have a number so how would the gate staff catch these?