I can vouch for the GAPAN tests. They're not pretty but they
are validated, and it's the stuff (scaled down to a half day) that the RAF use to assess pilot suitability, and have been in one form or another for 60 years. ish. I thought they were bloody good fun, and truly impartial testing. You can't revise for them, and when you're doing them you have NO idea whether or not you're doing well. I found the debrief extremely useful too.
I think aptitude tests offered by the big schools are not entirely the crude marketing tool that some people allude to (after all, they have to make sure they can publish good pass stats), but I think there is some element of that. I don't see what use placing an excited teenager in a large jet sim would be, except to further excite said individual into parting with cash.
And as so rightly pointed out by student88, it all means nothing if the CAA say you ain't fit to fly. In fact, I went and did my class 1 before even thinking about anything else (wonky eyes
but not that wonky, apparently
)
Cheers
JR