Well, firstly let's be brutally honest - a degree in a social sciences subject such as Political Science is of virtually no use, and makes little difference to your employability (except maybe in a few esoteric areas such a political research or aspects of journalism and diplomacy). It also absolutely is not a science, and shouldn't be called that - which is just offensive to real scientists.
You are more employable with just A levels, since you are just as useful but with lower salary expectations.
If you have maths and physics A levels at a reasonable (B+) grade, then you have potential to switch to Engineering. This is a solid numerate subject that gives you reasonable skills that are genuinely transferrable.
Is this of any use to you on a commercial flight deck? Not really no.
Is this any use to you in your future professional career? Maybe, since particularly an MEng with its significant management content may help you, particularly after you've achieved a Captaincy in 10+ years, develop a management/technical career within the airline.
Also, if your degree is in aeronautical/aerospace/aviation engineering, it *may* be a valid backup since you can show a continuous aeronautical career between flying and aeronautical engineering if you move into the engineering side of the industry.
Any other degree is of little use as a "backup" because you'll almost certainly have lost most of what you've learned and show no career continuity. Don't bother, it's a waste of 3 years and £40k. If you want a a backup, qualify as a short order chef or a lifeguard. Quicker, cheaper, and more employable.
Most TPs have an engineering or science degree these days, most commonly aeronautical engineering. But pretty much all "Heavy Metal" TPs at companies such as EADS or Airbus are ex-military. The other route into being a Test Pilot, in the "lighter metal" worlds such as Cessna or Learjet - the pilots were almost always Engineers first who did flying licences, not the other way around. And by Engineers, I mean people at CEng level plus, not just a first degree, which is only a career entry qualification.
G