My experience of T5 in early afternoon as a non-EU passport holder is that the lines aren't too bad. You're most likely to encounter horror stories in the morning when a large bank of flights from Asia and the US lands every day, or later in the evening when U.K. Border Force starts going home prematurely. Any time after around noon I usually counted on 20-30 minutes to get through, including collecting luggage, in my pre-registered travelled days.