All UK Airports have shopping facilities both before checking in (Landside) and after checking in (Airside). Landside facilities will invariably include Newsagents and Cafe's etc and at all the larger airports you will find small convenience shops selling a limited choice of groceries, aimed at both traveller's, visitors and airport workers. (Handy to get things like bread and milk etc, when you return home).
Once you are airside you will find in addition to Newsagents, Cafes and Coffee Shops the Duty Free Shop. Their size and the range of products sold will vary depending on the size of the Airport. Although they are "duty free" in the UK the prices charged for cigarettes and spirits, although considerably cheaper than the high street prices, can still be quite expensive when compared to duty free shops at airports outside the European Union. For example 200 cigarettes in Dubai can be bought for as little as £6. UK airports charge anything from £20 upwards. In addition if you are travelling to an EU country you may not be permitted to buy duty free alcohol and tobacco products. This is controlled by you having to produce you boarding card at the till.
You should also consider whether you are going to consume the items whilst away or are you going to bring them home with you? Remember if you bring them back they count towards your Customs allowance of 200 cigarettes and 1 litre of spirits, (when arriving from a country outside the EU), even though you may have bought them in the UK in the first place! So it.s worth checking whether the items are cheaper in your country of destination. It may be worth hanging on till you get there.
You should also bear in mind that most countries have their own restrictions on the amount of cigarettes and alcohol you can bring with you. Always check if you are unsure. Some countries are more zealous than others in enforcing this!
Also see: Shopping Abroad