I dont think you can do this if you do PCP, only if bought outright I believe as the car is not owned by you/your company until you pay the whole sum off. However, Im sure you can put PCP payments in your expenses instead but its best to speak to an accountant really.
Essentially, yes, whatever the Writing down allowances is (with personal use accounted for) is taken off your profit and therefore you dont pay tax on that. So yes, 20% off list price. for xxx number of years before the car dips below a certain value.
With the 100% allowance, you can only claim in the first year. With the 18% pool you keep claiming until the car falls below a certain value.