100% agree on the trade suggestion. Speaking as someone outside the trade, plumbers seem to be in especially high demand so you should have no problem getting work if needed, especially if you have builder friends or letting agents/landlords who will subcontract out jobs. In fact, you'd probably be able to pick up work in the winter to keep your eye in when things are typically a little quieter in the aviation business.
You'd probably be self-emplolyed, so if you needed to fall back on your trade for whatever reason, no awkward questions at job interviews about you going back to being a pilot as soon as you're able.
As for the cost, like all things it keeps creeping up. You'd probably want £60k minimum if doing it today + living expenses, but wouldn't be surprised if if are another few thousand as the years move on.
A dual licence would be sensible in order to maximise employment options.
I think you'd be lucky to get a £100k unsecured loan. Banks will usually want some kind of collateral for that kind of sum, so best to get saving. Plenty of people have parents that will re-mortgage their house, but getting the money together yourself definitely demonstrates enthusiasm.