The UK used to do exit checks until the 1990s.
The desks at LHR T3 are rarely manned (happened to me in 8 or 9 years ago). It is more common for officers to stand at the gate and do checks - twice in past 2 years
Most of my times there, they are not staffed for exit checks there. But I've had at least a few occasions the last 2-3 years where they were staffed at T3; most of those times last year, they weren't flagging down most passenger just after security. But a couple of years earlier, I had seen them there when they were flagging down everybody for a period of time.
I've seen officers at the gates at T3, but that is rather rare to see for departing flights unless you are looking for it, your route network includes a flagged route, or someone else is flagged.
A lot of what we see (and don't see) as passengers relates to the flight patterns of the passengers making the observations.
I've seen both kinds of exit checks at LHR last year. I've not cared to try to have my flight records from T3 last year matched with my observations of exit checks from last year, but for me both kinds of checks still happen from time to time but are rare. What I've not seen on my flights is checks both at the gate and just after security screening at T3 when originating the trip at T3. But that has happened to some others.