In a few countries the duty free shops are directly accessible on both departure and arrival. However, to my knowledge this is rarely if ever the case in the UK because passenger management means that those arriving in the country are separated from those in transit long before getting near the "holding area" where all the shops are. Passengers on flights with airlines who sell their tickets as individual journeys and provide no guaranteed connections (easyjet, Ryanair, etc.), and where you must collect and re-check in hold baggage at intermediate stops, must and are all directed straight to the arrivals hall. The main reason for this is that the UK requires all passengers from abroad to pass through immigration and by definition those arriving on a utterly non-connecting flight are arriving to enter the country. The UK has an open border only with Ireland. If you are in transit you effectively never officially enter the country so you go to the "airside" area and from there away again.
The reply you have on prices is correct: Even where you buy duty free you are not necessarily buying cheaper because prices at airports are so high - you may not pay the taxman but the airport and shop operator between them take an enormous cut from what you pay. I often use Toblerone as a measure of the level of rip-off. I recently passed through Zurich airport and there the chocolate was priced at around three times the rate per 100g offered by Poundland. Toblerone is of course made in Switzerland.