I'd keep an eye open in the supermarkets too. The Founders Reserve is one of those bottlings that often get sold for between £18 to £25.
Sainsburys currently have a nice Bunnahabhain at £25 and Tullibardine Sovereign appears at that price on occasion too.
Highland Park, Old Pulteney, Laphroaig and Dalwhinnie are notable in this price offering too...
And if you are ever in a place with a Booths supermarket, check their offering too. Scored a bottle of An Cnoc for £25 in their Keswick branch recently...