Sky - most of our supermarkets will not sell a can or bottle of generic lager - they are packaged in fours usually, or boxed in twelves. But "special" beer is sold in bottles and is always sold individually.
It sucks if it is available for sale but the cashier does not know how to process it - maybe write a note to your local supermarket explaining the training need.
Alternatively, go to an independent store. I assume you have off licences there?
They're happy for any sales they can wrestle from the giants, deal with smaller purchases and usually know what they're doing (ie offering tasting nights and good suggestions).
I lived literally round the corner from a place called
The Offie in Leicester.
I didn't buy all my drink there (when I was drinking too much I went to Bargain Booze) but back in the days where things were under control I was a weekly visitor for a couple of foreign beers on the weekend.