You could buy a bottle of Teachers and serve yourself 25ml for 70p. You could buy a bottle of 30-year aged Macallan matured in sherry oak, and serve yourself 25ml for £160.

In a pub, you should be able to get a shot of blended whisky for about £4-6, and mass-market single malts (the kind you also find in supermarkets) for about £5-£10.

If you don't ask for a specific whisky, they should ask you which one you want, and/or say "is name alright?" and give you one of the cheap ones.