Price fixing is illegal in the UK so dealers can charge whatever they can get away with and whatever the punters will pay them! It's a competitive old world for them so usually they will come down if faced with cheaper prices. However, once the deal is done...
There's nothing to stop you going to them and arguing that they've ripped you off but they might have given you the best deal going at the time you agreed the sale.
Watch out for some internet dealers however, who can't necessarily deliver what they offer.