A good aero degree is an obvious start - which you're doing.
The next thing, in my opinion, is that you want flying experience. It generally doesn't matter what in - light aeroplanes, gliders, microlights are all fine (and helicopters, if you're rich, or gyroplanes, if you have a deathwish!).
Then it's very much a case of getting a bottom level job somewhere with an FT department: Qinetiq, AgustaWestland, and BAeS Military are the three main employers in the UK at the moment with large FT departments, and all have good graduate training schemes which *can* lead in from the bottom. And by and large nobody goes into flight test anywhere above the bottom.
So basically you want to position for a graduate training post in the first instance, and then once you're in there, for a junior position in flight test. Neither is easy, and competition will be stiff - but somebody has to get there. There will be elements of luck, and very much of ability, but determination is a massive factor.
In the short term, in my opinion, there are three things that you can do to help yourself position for where you want to be:-
(1) Get some flying experience. Money will be tight, but try the university gliding club as likely the cheapest option.
(2) Join SFTE as a student member. (See
SFTE Home ) [And try and get to some lecture(s) or symposia, ditto RAeS flight test group, who usually hold their meetings in London.]
(3) Try and position to do your individual dissertation with some flight test content. This may mean sweet-talking an appropriate lecturer, and most likely some significant legwork yourself, but will be massively useful when it comes to job interviews in 1-2 years time.
Postgraduate study *might* help, but probably only if you can't manage to get onto a graduate scheme and into a big FT department at the bottom. If so, the best bet is either the Cranfield Flight Dynamics MSc or the NTPS Masters in Flight Test in Mojave - but neither is cheap.
G