It isn't if you have the certificate saying you proved you can't afford this sort of thing. Most NHS dentists have the paperwork and posters advertising it if you are somehow unaware those exist. Everything is free if you're both poor AND fill out all the paperwork.

There is a debate to be had about whether "make poor people fill out paperwork" is a justifiable approach, but it doesn't have to cost you £20 for the check-up.