My two cents worth... I dont think any public examination or degree in the vast majority of subjects is a reliable guide or quantifier for ability to do/train to do ATC. The skill set needed is very specific and i would suggest unique to the job. I know various people who have 1st class degrees from Oxbridge and the like who couldnt begin to do what we do, but then again I know people with minimal A-levels who might well be able to.
The criteria that NATS look for may or may not be right in any one person's opinion, but they seem to be doing ok at it. Ok maybe cubes might not have much to do with ATC but they display an ability to be spatially aware and process information quickly and accurately, which is an obvious advantage.
When all's said and done if you get through to the college its down to you. If you can do it, go and do it, and if its not for you then so be it. But either way you gotta work hard to find out
Thats more than enough of that...