I bought an 06 RS4 Avant with 72k miles last week from a dealer here in Northern Ireland for £19.5k. Having looked at all the car available around the country low mileage examples seem to command strong money, enquired about a 58k mile car selling for £23k!
At the end of the day if it's clean and you really want it, buy it!
A car is only worth what somebody is willing to pay for it. They know that and there is always some wiggle room in a price usually at least 5%.
There are plenty around the country, with cars like this you are unlikely to find just what you want on your doorstep. http://www.autotrader.co.uk/classified/advert/201609228028965?