Surely you will be doing a full plans submission, and the plans will be drawn up by someone competent?
What actual work would be detailed on plans which a plumber could not aciheve without plans?
Dimensions are not critical; there's a 4" pipe to connect to the existing drain via a small chamber; a waste pipe from the wash basin, a fan linked to the light switch.
Toilet-roll holders, soap racks and door locks don't come under building regs as far as I am aware.