Make sure you sit as far forward on the outbound as you can. Even when arriving behind 2 non-EU arrivals I've been through passport control in less than 15 mins via EU line when first off plane (but summer can be busier...), and fast track *should* take less than 10 mins. With a brisk walk well under 5 mins back to gate from there.

It is tight though. Best way of de risking (IMO) is to take last flight night before and stay at airport hotel. £60 or so extra, but gives a plan B. Personal preference though, and LCY handy for me from work or home