You can often get a bottle of Aberlour (Speyside) or Jura (peaty) in Spar etc for less than £20. Otherwise it's £33 for a bottle of Laphraoig at Tesco. Assuming you have Spar and TEsco in Pimlico. Didn't you used to need a passport to go there?
There's supposed to be a website somewhere where you can find out which distilleries make which supermarket "own-brand" malts but I'm happy to trust my palate.