Do it now, or you'll never do it.
Suck up any money lost and do it full-time, you'll get the hands on experience you'll need that colleges don't teach these days. We've got a young lad (16) started with us and his one day a week at college is taken up by H&S courses of learning how to lift boxes, and maffs and English tests. I kid you not, the kid learns nothing about plumbing there.
The building trade is still very quiet (more or less), so you may struggle to get work right now. That said, decent Part P sparkies can charge what they want, it just costs a load to get there in both money and effort.
Also, and don't take this the wrong way, but £12K a year is shockingly bad money. I'd be doing something I enjoyed at the least if I was earning that little a year, so honestly you don't really have much to lose apart from not having a job at all.
I wish I'd done it years ago truth be told, I'd have loved to be a copper of any kind but there's no opportunities for that now.