It largely depends on the calibre of the agent and how they put the offer forward. I offered 20% less on a house recently, the vendor was insulted but the agent persuaded them to make an offer on the property they wanted and see if it could be made to work. That property ended up falling through, so subsequently mine did.
At the moment, nothing is shifting and everyone thinks their own property is worth more than it is. If they can haggle a similar drop along the chain, then things will keep moving.
Arm yourself with LOTS of comparables, nethouseprices and zoopla are good, but as was mentioned earlier, always start with a positive: “We love what you’ve done with the decor, we’d be happy to move straight in as it is… But, similar properties have sold for a lot less” etc etc…
(My brother is a regional manager for a large estate agency, so his coaching was very useful throughout the house buying process!)