My wait time was longer than SgrA. Last time I was at Heathrow was before COVID19, and was traveling with non EU passport. Line had at least 150 people, but some those people travelled together. For them, more than one person cleared each time.
6 separate immigration counters (one officer at each counter) were running, but I waited at least one hour in that queue before seeing officer.
Next to line is waiting area or penalty box. I saw immigration officers asking people seat there and wait. Then officers went to back and went to some door and disappeared, probably to investigate things? After they are done, they come back and fetch you in waiting area. Thus if you have to wait in penalty box, you will wait even longer!