It sounds decidely un-British, but then things have changed in recent years. I find it hard to believe that there can be a difference of £500 between the advertised price and the actual price paid. How much was the advertised price? More than £600 I hope.
Anyone buying goods should only pay what they think its worth to them. If the price is too high they can nearly always go elsewhere, so I don't see any need to haggle over price.
Uncertainty over pricing has caused a lot of negative reaction on the Serif Forum, with some people paying significantly higher prices than other customers. For the retailer, I think it is a bad policy, as it looks like dishonesty.