APD would be £156 at the moment, increasing to £172 next year.
https://www.gov.uk/guidance/rates-an...passenger-duty
Most of which can be avoided by a 24 hour layover in Europe on the same ticket, or buying two separate tickets for a same day connection. Many airlines such as KLM, Finnair, Lufthansa, Air France could be suitable.
An open jaw ticket on Thai Airways from Europe to BKK and back to LHR might be worth considering. Eurostar to Paris or Amsterdam and then flying from there can also be a major new saver. A rail/sea package to Dublin and a connecting flight on one of the Middle East Airlines could be an option.
Fly from somewhere outside rip off Britain and save your money.