Thank you, both for your reassurances! I appreciate it!
Does anyone have a recommendation between (a) clearing security as a transfer passenger, possibly without fast-track, versus (b) entering the UK and then going through fast-track security as if my travel originated in London?
How much time do you have and when will you be connecting? These factors can make a difference. See the relevant section of the Guide (First Wing is currently closed for remodeling):
https://www.flyertalk.com/forum/25935798-post8.html