Boltzman,
We have a near identical background, I'm an aero graduate, hold a an instructors rating and like yourself funded my way through my fATPL while working in the engineering industry. Last summer I was over the moon to be offered an airworthiness position with a certain JAR 23 manufacturer, with the the stong possibility of doing FTE work (on the grounds of having completed the Flight Test Short Course on the Cranfield Jetstream as part of the degree). However it was made clear very quickly that only having single engine piston experience as a flight instructor there was no way I would progress to the left hand seat; furthermore although essentially a civil aircraft due to the fact that nearly all projets were on a military/annex 2 register then there was no way I would have been allowed to fly anything. The problem with the UK is we no longer have any complete civil airframe manufacturer hence this makes it near impossible for "civil" pilots to progress into flight testing.
You may get into flight testing at the PFA level, but you will soon realise that once you have invested the money into a frozen ATPL you can't afford to hang around in permanent engineering jobs waiting for an opportunity for FT work. Also the airlines will give you very little credit for the engineering. That is my experience, I am now concentrating on getting a run of the mill civil flying job.