This is a problem that it seems we all have to deal with. I gave up negotiating my prices, I quote a price that is reasonable but make sure that I can make a living, otherwise I will seat at home and watch "Stargate"...
I spend a lot of time, during the first visit, to explain the customer about part P and BS7671, I explain what is involved in doing the job, for example; if a new CU is involved I say that I can get for them split load CU (and emphasize that it is safe) from B&Q for less than