I am on a bit of a personal crusade to try and get some British universities to offer distance learning first degrees in aeronautics. At present I don't believe that there are any, although I do know somebody who did one part time at Farborough Tech. Embry Riddle in the USA certainly do, and have a pretty good reputation - although it's not cheap. If you find anywhere in the UK doing distance learning, please let us all know because I know a few people I'd like to steer towards one if such exists.
You can do a general Engineering degree with the OU, which is probably the next best thing but lacking things like Mechanics of Flight, Aircraft design, aerodynamics, etc. wouldn't be ideal. I suppose you could try and do that and persuade them to let you take some specialist courses in a fixed university - anything's possible with enough determination. I can certainly put you in touch with a couple of good aero-eng departments if you wish (specifically Southampton and Glasgow where I've reasonable contacts).
Bristol University does a distance learning MSc for aerospace Engineers, you could always see if they'll allow your ATPL + some kind of foundation course to get you going? (They call it IGDS, Integrated Graduate Development Scheme, or something like that anyway).
Experience on a wide variety of types is essential. You're unlikely to find many civil TPs with less than 25 types in the book, and 2-3 times that isn't unusual. Even in the military, pilots need captaincy on 2+ front line types before the system will consider them for TPS. (Looking at my logbook, I had 22 types logged as pilot or FTO before I did ETPS, and that was for the Engineers course !).
G