In my experience in air traffic I have seen folk of high and low abilities (out of the college and from other units) validate at traditionally difficult places such as Heathrow and Gatwick easily and struggle at traditionally quieter units (won't specify, don't want to offend). I personally think that the quality of OJTIs matter more towards validation than raw ability. If someone reasonably competent from Oxford or other not 'busy IFR big jet' units go to Gatwick then I think a few good OJTIs should validate them. Not a view shared by everyone I'm sure, and I stand ready to get slammed by someone saying I haven't worked at a top 2 in Britain unit therefore I don't know 😋