Some more details:
Aircraft involved was BA A321 G-MEDU, operating BA777 (BAW77C) from Stockholm/Arlanda (ESSA). Inbound via Lambourne, twice round the hold there and then vectored as normal for a 27L approach. Airprox classed as Category A ("Risk of Collision: aircraft proximity in which serious risk of collision has existed")
Links:
BA777 20160718 Flightpath from Heathrow WebTrak
UK Airprox Board Report 2016139