The law clears it up nicely, but it should only need a bit of common sense and ethical prowess for plumbers to quote correctly.
If the plumber had never in his working career come across the problem of members of the public concerned about the vat price confusion, it is because he correctly includes the vat and probably had decent parents who brought him up to work out right from wrong.
Plumbers that hide the vat on an oral vat net quote and then pretend the customer is fussy by questioning why the extra vat was only mentioned after the job was done is the sort of rip off cowboy plumber probably brought up by slap dash parents who spoilt the little brat.
Apart from the parental up-bringing theories, it only makes sense to mention before hand the price of the job and any rate of tax being applied to avoid any confusion or embarrassment by either party after the job, its only logical and polite.