An agent can advise the seller that for a quicker sale, a house should be listed at a lower price. If it is to the agent's advantage to make a quick sale, the agent can reduce the commission by whatever amount s/he chooses to help induce the seller to agree to a lower asking price. As long as this is done openly (to the seller) this should be legal.
In some jurisdictions the agent's actual commission must be listed in the closing documents. In those cases, such rebates would probably have to be listed. I don't know if this is true anywhere in the UK.
Note also that "market value" is not an absolute, objective figure. it is merely a prediction of what price a seller and buyer will agree to. For unique goods such as real estate, it is even more problematic than cases where many identical widgets are sold at a fixed price. No two houses or lots are quite identical in the qualities that make them of value to a buyer, so there is never a guarantee that a sale will be at a similar price to that of a "comparable" property.