Main Dealers charge roughly 25% more than trade value to cover costs and warranty. There are private traders in the UK that sell legitimate 1 owner fully serviced vehicles, this is the way to go for BEST value.
Before doing the deal make sure you HPI check for outstanding finance, insurance claims and possible service history. Also, make sure the service manual has plenty of stamps in the book at the required intervals (make sure Timing belt has been done if over 60K miles) It is good practice to look a the last stamp in the book and ring the garage to confirm their record tallies with the record in the book.
If the last service in the book is over 18 months be careful, the car may be clocked ! Clockers look for these opportunities.
Depending on location of your vehicle registration office (VRO) they may inspect. In the case of an S-Line, if the standard spec is leather, alloys etc then this has already been factored when calculating VRT, it should not be considered extra.
Revenue do carry out sample inspections and may audit after VRT has been paid (I am told).