I did this
1. Google "40 biggest Motor dealers" or similar (Uk sites only) (big dealers - big discounts)
2. Look at each and find all those who flog SEATs
3. Check all the SEAT special offers out
4. Make a list of the cheapest ones and phone to confirm prices
5. Go to local dealer pre armed and make a deal.
So I went to Pentagon to buy a 16.5k car but only willing to pay 12.6k for it, based on phone quotes from Vertu/BSM and Arnold Clark. Pentagon had them online for a PCP price of just over 13, but I got the price-match for cash and ordered one there and then
It's possible that online brokers will offer lower prices, but I couldn't be bothered waiting ages for delivery when they were only, at the time, £300 less at best and I didn't want to possibly get lumbered with something older spec out of stock.