Thanks HogwartsExpress, are all the Fast Track lanes across terminals (as shown in the picture below) now closed? Last I flew into LHR was in April 2023 when this lane was still active (I may have incorrectly thought this was active) in T2 (but I flew on LH so no Fast Track anyway).

Edit: Looks like Fast Track at immigration is no longer a thing, but was briefly brought back for a 3 month trial in Jan 2024, as this article highlights.