WWW,
Just to set the current record straight regarding people getting chopped here in Jerez. Since the college introduced compulsory entry testing for potential candidates or the requirement to sit the GAPAN tests, no one tested has been chopped.
I did witness 3 people getting chopped mid last year (who all started prior to those entry requirements) and they included a chap in his 30s, a young lady in her early 20s and a young lad of 19. No obvious conclusions from that cross section.
So do the GAPAN tests wherever you choose to go. If you choose to kick it off with a PPL, do the GAPAN tests before you do anything - medical, introduction flight, whatever. Alternatively, if you are set on Jerez or Oxford, do their entry tests.