Instead of thinking it’s from a warbird or commercial aircraft, how about a GA type? It certainly looks strong enough for that.

Something along the lines of a side window for a Rapide or Hornet Moth (though I’m not specifically suggesting those two types).

I’m pretty sure it’s not from an American GA aircraft, and the UK doesn’t leave a lot of options, so perhaps something from the continent?

If that doesn’t work, Id suggest a caravan or bus.