There is no global selection process for ATSAs in NATS. When external recruitment does take place [which is rare these days] it is done locally by the unit that needs an ATSA [or ATSAs]. This only happens though if internal recruitment is unsuccessful.
Best bet is to send your CV to the Manager ATC at the unit you wish to work at.
Minimum qualifications are "ability to walk upright" and "possess opposable thumbs". It is more dependent on what sort of person you are [accuracy and an ability to learn are more important than academic prowess at school].
If you post here where you want to work, maybe some kind soul will PM you the details of who to send your letter to....