Customs is non existent in the UK, you would have walked past it in Flight Connections and probably not noticed it (clue: it's a small phone on a glass wall); going landside it's slightly more noticeable as the morgue like area between the carousels and landside. Except there is more life in the average morgue. This may change at some point.... But moving swiftly on, just go out to landside and look for the olive green or orange lift (elevator) shaft by the Costa café and take that to the top floor, departures, then swing rightwards when facing the check-in area, over to far end of South, as if going towards Zone A, and the semi-wall for the First Wing.

There again, the USA is slowly moving to a similar position where CBP handles 95% of customs too.