I appreciate everyone's responses and thoughtfulness for answering!
I ended up changing the pick-up time to 3pm.
I've never rented from Heathrow (or driven in England!) before and had been wondering about the various steps from the gate to the rental car counter, which someone helpfully addressed (and which I should have clarified was my primary question).
My main rental car experience is from the Atlanta airport (Hartsfield-Jackson) where there is a train involved in getting from your gate to where you leave the airport and then another train to where the rental cars are. Not hard to do, but takes some time to play out.
My experiences with Heathrow have been really stressful in the past! It's my least favorite airport, though the staff were lovely. I've not flown internationally since maybe 2004 and so I can't remember all the steps... I have never had to wait long in line for customs but it seems wise to anticipate/prepare for a long wait.
I was thinking about why it would be stressful to drive in an airport area and I feel like yes, the signage is usually confusing* and then people tend to be stressed, tired, and even angry getting off the plane, waiting for luggage, waiting to be picked up, and once they leave the airport might not be the most relaxed and patient drivers. (The states of mind of my sister and I after flying from Atlanta to San Francisco with two young kids, four suitcases, and four cats was not stellar.)
*I am in the San Francisco area and still get confused leaving SFO, though I've done pick ups from there dozens of times!