With regard to the UK "test flying " for a civil trained pilot would be limited to post maintenance test flying and fleet acceptance testing as a technical pilot for an airline.
There are so few manufacturers of complete airframes that the test pilot work in the UK (with the exception of PFA type aircraft) is in hte realms of the ex military. I have heard at Bombadier in Canada it is common practice for Flight Test Engineers whom have completed flight training to progress onto test pilot careers.
If you are interested in test flying and have missed on on joining the services then consider a career as a flight test engineer, if you want to be a commercial pilot go train as a pilot, but if you want to mix the two then unless you have Canadian or US citizenship it seems near impossible.