The Estate Agent acts for the current owner, the seller/ vendor, who would pay them a commission (i.e. varies between 1.5% and 3% depending on where you live of the sale price of the house) which can vary considerably - depending on what's been negotiated.
The buyer does not normally pay a fee to buy a property through an Estate Agent. (For the Agent to charge both sides would be unethical). Estate agent fees in SW10 is usually regulated to commission. However it is in the agents interest to help you, as without you, the buyer, there's no sale and no fee to them.
People in a hurry sometimes offer to pay a higher fee (or more percentage) if the house is sold within an agreed time limit.
Estate Agents can make extra money from you - the buyer - by offering to arrange your mortgage for you. Estate agent fees in SW10 is usually regulated to commission. Remember that they actually represent the seller not you so that's a grey area to start with.
However the real reason to be careful is that they can coerce you into accepting "their" mortgage by threatening at the last minute to give the property to someone else. So you might end up panicking and taking a much more expensive home loan than necessary. Estate agent fees in SW10 is usually regulated to commission. OK most Estate Agents aren't that bad but apparently it has been known to happen.
If offered this "service" by the seller’s agent you're probably best off making it very clear that you already have an independent mortgage broker on the case. Estate agent fees in SW10 is usually regulated to commission. You could innocently query the legality and their Trade / professional codes of your accepting this service from them - i.e. don't alienate them early on by putting them on the defensive: Put it in terms of mightn't you get into trouble. Estate Agent Fees in SW10.