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Plumbing is worthwhile but it takes time to qualify and costs money. The best deal I've found so far for gas safe qualification was £6000 and 6 months working for free. The electrical course I took was £1800 and you can go from zero to 3 c&gs in 4 weeks, give or take scheduling. Of course it's only a small part of the trade, mostly focussed on designing and testing new circuits. Not cable pulling etc.
I started so long ago it's a faint memory. Probably the best way to start is as an apprentice/helping out a mate, if you feel you're too old for that then you can take on work for less money than market rates and take a bit longer over it until you get experience and tools together. I still spend at least £1500 a year on tools to make life easier/improve the end result. I've never really worked on building sites, mostly domestic and that's the market most trades want to avoid. It's very hard to recommend as an occupation except in terms of job satisfaction, that part of it can be rewarding.
Doing a course is a good introduction to the whole process though, if you can get all the way to gas safe qualified then there's good money to be made and you're working in a trade with some protection.
Snowed under at the moment, I've got 12 months of work lined up! Been turning down 2 jobs a week since the start of January. I think it shows how the market has changed in London, teams of trades doing renovations and no-one wants to touch the smaller jobs.
I did just take and pass the exams for Part P and BS7671 though, that's why my electrical fu is strong atm.
Light sockets throw most people, the colours don't mean what they usually mean and half the time they're not sleeved to let you know they could be line conductors. Never assume that a blue or black wire in a light switch is neutral.