Clearprop,
I am NOT a pilot but I've been active in Flight Test in North America for over 25 years (until my recent retirement). As I said, it's a very small world and even ex-mil pilots won't have an easy or automatic entry into a Test Pilot slot. There was a previous thread on just this subject (unfortunately recently deleted) and my impression was that the TPS short courses were probably not worth their cost unless they were backed up by actual practice (a vicious circle if you're not already in Flight Test). I would suggest that Genghis could provide much better insight than me on UK/Europe courses as could our Moderator, Pilot DAR, for North America.
My advice remains to join the company you're interested in first wherever it may be.