No they don't. They charge 40 quid for:Well the plumbers round my area charge £40 labour for 20 mins work.
The time it takes to get there.
The time to do the job.
Small materials. ( a roll of solder is about £15 these days, and a tub of flux the best part of a tenner)
Overheads like commercial transport, insurance, tools, compulsory membership of bodies like GSR, reservations for tax, holidays and sickdays and a number of other things.
Finally, included in that is the guarantee that the job is done properly, and IF it would fail, the damage won't cost you anything.
Compare to your own situation; the money you cost your boss is a lot more than what comes into your bank account at the end of the month.