Hi Paul,
For the tax issue, firstly on the sale of a new car there is of course with many things 17.5% vat (value added tax) but the buyer pay the tax NOT the dealer. Then if you part exchange the car with the dealer, the dealers does not pay vat on the amount they bought the car for. If the dealer then sells the car later they only pay vat (17.5%) on the profit not the total sum this is called the margin scheme.
So for example if you part ex a car and the dealer gives you 10k for it, the dealer then sells it for 13k, making 3k profit they pay £525 in tax for that profit (3,000 * 0.175). The other tax you mentioned I guess is company tax, which all businesses in the uk have to pay but again it's on profit not on turnover.
Using the tax situation to to justify giving customers a bad deal sounds a feeble reason to me, as you only pay tax proportionally on profit not turnover so charging more does indeed mean more tax but it also means more profit. Also you can offset the profit against running costs like your the rent of your premises etc. I learned the small specifics to automative taxation because I owned and ran a car import company for years and did all my own books etc.