ITPS (if they are still in business) and ETPS also offer short courses in the UK, but none are cheap. Also, I'm quite certain that ETPS knows very little about light aircraft testing, and ITPS not that much more.
The CAA will accept the training of any of these schools, or the French school EPNER in the majority of roles for TP/FTE. However, it's not really something that you can "self-improve" through, the costs are VERY steep.
If you want to get into the field, particularly in GA, the best way is to get a job within an organisation that does that sort of work, and learn on the job. That is how the majority of people working in GA FT came into it. However, this is a tad off-track, so I shall stop here.
G