thats the trouble with technically skilled trade you can only earn your hourly rate x your hours but with a skilled trade like plasteres who quote by job / meterage can earn proper money.
yes you can be a self employed sparky and quote a job get stuck in and earn some but for some reason ppl really do think sparks / technical trades are pulling a fast one, cus sometimes you can't pyshically see whats being done
i.e a big factory a spark installs a long run of trunking which you can see progress(work) as each length goes up. but then it comes to wiring and all the cables could be going in the whloe length of that trunking but you don't see any pyshical erection of any materials going up where as most other trades you see progress at every stage ,i really believe this has summat to do with the skilled trades not enjoying as rich a pickings as other visual (yes agreed full skilled trades) but not technical in general.
go on any big site 2 trades will guaranteed be paid by the hour sparks/plumbers the majority of the rest on price