It's mainly luck
i know several persons involved in Flight testing here in the South West, virtually everyone of them has said it's all down to being luckily in the right place at the right time....
Qualifications can be from in house Apprenticeships (then onto BSc) up to MSc levels, but most of the training seems to be 'on the job' (certainly outside QQ).
I suggest you keep persistent and keen but it is a very small community in the UK (Genghis has stated that there are 'large' FTE communitites in the UK, for that read less than 20 for any one company, we are not talking hundreds of jobs out there)
good luck with the hunt
DM