You can't negotiate. The internet has forced car sales to display their "negotiated" price on-line. If the car is advertised at a more expensive price than the competition, then the phone doesn't ring.
The showroom makes up for it's losses through add-ons, extended warranties, break down service, "free MOT's for life", that sort of thing.
It's a myth that you can get a discount on a car these days.
My husband is a sales manager for a very well known car maker. I have spent 15 years listening to the in's and and out's of the car sales business.
Pay the sticker price. Don't buy any extended warranty, they're not worth the paper they're written on. Don't accept their offer of a free MOT either. Your car will "fail" and you will be at their mercy, thanks to the new MOT rules.
You cant get any discount. If any salesman offeres you one, there's something wrong with the car and they are trying to get rid of it.