Chain-free cash buyer — how much below asking is reasonable?

I'm a cash buyer, no chain, and can complete in 4 weeks. Looking at properties in the £400-500k range in Surrey.

How much below asking is reasonable to offer given the strength of my position? I don't want to insult anyone but equally I know cash and speed have real value to sellers.

Agents keep telling me "offers in excess of" but surely that's just marketing speak?