Terminal 3 definitely has a reasonably sized M·A·C "store". I'm not sure if it has the full range of a regular M·A·C standalone store though. Other terminals may sell M·A·C within the main "World Duty Free" area.
You can contact World Duty free at Heathrow on the following numbers to ask about whether they stock M·A·C;
Terminal 1 - 020 8745 0282
Terminal 2 - 020 8745 4148
Terminal 4 - 020 8745 5161
Similar contact details for Gatwick;
North Terminal - 01293 507301
South Terminal - 01293 502798
You can also email World Duty Free to check on stock at
[email protected]