Never experienced this in LHR, but was almost stuck in a lift in AMS one time (door only opened a couple of inches then stuck and refused to open or close. I managed to force it open which was lucky because there was a real chance I could miss my flight even if I'd been stuck there for just 10 minutes.

One thing I noticed about T5 from my first visit (back when they were doing the terminal tests) is there's a lot of unused lift shafts. Were they planned to be unused at the beginning and extra lifts added later when demand warranted it or were the number of lifts installed reduced as a cost cutting measure?