An airport departure tax of £73 (£13 for within U.K. and EU countries) per person is included in the price of your ticket.
The British sales tax, V.A.T. (Value-Added Tax), is 20%. It's almost always included in quoted prices in shops, hotels, and restaurants. The most common exception is at high-end hotels, where prices often exclude V.A.T. Further details on how to get a V.A.T. refund and a list of stores offering tax-free shopping are available from VisitBritain.
When making a purchase, ask for a V.A.T.-refund form and find out whether the merchant gives refunds—not all stores do, nor are they required to. Have the form stamped by customs officials when you leave the country or, if you're visiting several European Union countries, when you leave the EU. After you're through passport control, take the form to a refund-service counter for an on-the-spot refund or mail it to the address on the form after you arrive home.