I used a broker 10 years ago when I bought my X-Type. Local dealers couldn't get near the price so I ended up travelling to the main dealer (HA Fox in Guildford) to pick up the car. Can't remember the broker name, but they closed up shop - not sure why. It was owned by Inchcape, so not a fly-by-night jobbie.
Two years ago I used orange wheels (after suggestions from here) for the XJ and my local dealer (Rybrook Chester) were very very close to the broker price, so I went with them.
I think the difference over time is that dealers know that the brokers are out there and so are much more inclined to give the best deal they can. I got very short-shrift 10 years ago, whereas all they did this time round is double check they saw the same on the broker site.
When the different was thousands, people will put up with the train ticket and travel hassle - when its down to a few hundred quid I reckon most will stick to the local dealer instead.
Brokers are nothing new. I remember adverts for them in car magazines in the 80's and 90's. What the internet did was put them online and force the premium manufacturers to play ball as well. I never used to see Merc, BMW, Jag etc covered, it was just Ford, Vauxhall and their ilk.