One more note: A confirmed-at-booking GPU should always be taken over a waitlist if the route is slightly more annoying.

I suppose this is a point of personal opinion, but there is a very real chance that a waitlisted GPU won't clear, so flying PHL-EWR-LHR vs. PHL-LHR is a minor inconvenience for a world of difference on the TATL flight.

I made some mistakes early on with GPUs because I thought most waitlists would clear. Very much not the case. Of course this does depend on what routes you're taking but any routes to LHR are usually pretty booked up in BF.