Claire, I agree with you partly, i.e. that the commission is included in the price. However the Agent does not say how much do you want and than add the commission.
Baring exceptional/unique properties the prices are within £5k, The agents knows the prices in the area from various sources. e.g land registry.
There is one large Agency chain in UK, and also in USA and are trying to break into Spain, who blatantly over value the properties, they dont get shifted and later reduce the price but not the commission rate remains which in the first place was higher than other agents.
They properties regulary gets down value by surveyour or they add a caveat. This means that if the buyer is borrowing 95%, the deal falls through.