I believe drive the deal are pretty much the best for deals right now, with discounts on not only the car, but specced options too. The broker will give you a p/x price if it is something that is desirable for them to sell on their used lot (they might not be interested in a 7 year old Vauxhall). They aren't going to give you a great price after pushing for 14% discount on the new one - worst case scenario should be WBAC.com valuation.
I have dealt with a broker before - ended up putting me in touch with Lookers of Burnley.
When are you expecting the new car? It is highly unlikely you'll see a new order this side of Christmas, whether you go retail or broker. If you go broker, you have more of a chance getting it sooner as fleet sales will probably have more generous allocation than retail.
What is your current car by the way?