Sorry Colin just couldn't resist it, knew I'd get at least one bite!
In actual fact the value of anything for sale is determined primarily by two factors a) what someone will pay for it b) what the seller will accept for it. The factors in here are generally determined by; for the buyer rational and emotional feelings, for the seller, the need to sell against the level of genuine interest and time available.
Regional variations can apply and I would say are greater across countries than throughout the UK. Local supply and demand on run of the mill boats affect it as well to a certain extent because of the cost and problems of transport, as does the availablity to put the boat somewhere. All basic stuff. Even new has no boundaries, I recently asked a broker with a new boat at £53k to look at the price again and he came back with £47k 'cause he can't sell it.