Several years ago departing LHR a British Passport agent was checking passports on leaving, they may have stamped my pp--just one time out of many trips out of LHR.
They used to do passport checks and departure stamps when exiting the UK as recently as the early-mid 90s. In an old passport, I have a triangular departure stamp from Heathrow in 1993, when I left following my first trip to London. However, I think they eliminated that procedure shortly thereafter.