There was a comment about them being incentivised by commission, I was pointing out how little they actualy make from one sale which would explain some of the arrogance being shown (in light of the deal), especially if the vehicles are over subscribed anyway.
I hate to say it, but on a £30k car, or a £130k car (from a volume manufacturer) you're not going to be treated like royality by anyone, most dealers are incentivised the same way. Fact of life. It's not a how much the vehicle is worth, it's how many units they shift, in combination with moving used car units as well. The franchise owner will be rewarded based on hitting these targets. That's how the salesman and the dealer make their true money and bonuses. I know because I have family in the trade and I employ ex-salesmen at my firm. The incentives are true for the majority of the volume manufacturers just the schemes vary slightly.
No matter what, this does not excuse how the OP has been treated, I totally agree it's just disgusting. However it may give reason to explain the actions of the individuals involved at the dealership, as distasteful as it might be.
This can only be addressed with VW UK you won't get anywhere with the local dealer now. Social Media is the best way of causing a stir if you don't get anywhere with them direct, but I would certainly pursue the proper route first.
I might also suggest that dealing with a Broadspeed or a DriveTheDeal removes any semblance of margin for the dealer on the car, so the only interest from the dealer here (from a business perspective) is on target/number of units sold and the miserly commission for a one off sale of a car for the exec. All of these items will combine to make the dealer less interested in your deal. Again, untenable from a CS point of view and this is why VW UK are your only route, that, or sit back and leave it to Broadspeed to sort out for you.