LHR is basically a shopping mall pretending to be an airport with an extraordinary percentage of space devoted to all kinds of shops. There is a main duty free retailer in all terminals with a big range of standard brands at prices which are lower than standard retail in the UK but higher than easily available in California. However, Terminals 1 and 4 have outposts of a specialist whisky shop and here you should be able to obtain some interesting bottles which are less generally available. As always with airport shopping, the prices tend towards the outrageous.