Portsharbourflyer,
There are 2 Brit test pilots with all-through civilian backgrounds.
Your other points above are all well-made, and I certainly don't underestimate the difficulty of making a success of this career from a purely civilian background and/or from an Eng degree start. That was my point in my last para above, when I suggested that InquisitivFlyer would need to be prepared to spend time or even complete his/her career as a design engineer, FTE. There is certainly an element of luck required. Those of us that got a flying start from the military do indeed have much to be thankful for, although that's a very hard path too! There is no easy route.
Someone mentioned QinetiQ, and I think they also still employ Flight Test Observers (FTOs), a position that has historically been a springboard to FTE status. The FTOs have typically been engineers and trials officers, whose work involves occasional requirements to fly. Airborne trials photographers are also in the FTO cadre.