Originally Posted by Gagravarr

You shouldn't need to go near customs, just immigration, unless the escalator up to north transfer security looks dreadful. Follow flight connections, then UK flight connections. Just before you get there, look at the escalator behind going up, and the queues. If they're really bad, go back and follow arrivals + exit via the baggage hall and customs (no need to stop), then go up stairs and enter through departures. If queues aren't too bad, go through UK connections immigration then upstairs and through the north security. You should be fine, as long as no major delays

Whatever you do, head to the North lounge for a shower. While there are theoretically showers in the elemis spa in the South lounges area, there is in practice only 1 wheezing and groaning and falling apart shower there, with the rest broken beyond repair, and 2+ hour wait times. Should've been fixed some time ago, but BA seem to not care... North lounges have recently had their showers mostly fixed, and generally has shorter shower queues (not hard!)