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.
I was going to do this when I was a student and the form was so long and convoluted I decided to just pay. It all seemed just slightly designed to stop you claiming it.
I filled that form out too. It wasn't the quickest, but it wasn't difficult either. Definitely didn't get the impression that it was some kind of barrier.
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.