What's the latest with Heathrow - Terminal 2 - for departing

We flew home from T2 on United last week. We arrived 3.5 hours early and there were no problems going through the steps to drop off a bag, go through security and wait for our flight’s gate to be posted about 1h before boarding started. We made sure all of our flight ready boxes were checked on our United app and checked in 24h before our flight, the last step. We could tell from previous United flights that our gate would most likely be in the wing of the terminal that is a 20 min walk from where the gate posting board is located. Fortunately, United texted us the gate # before it was put up on the board so we started the walk out to our gate.

As you've already noted, Global Entry means nothing outside of the US. Your premium seats will also have zero bearing on how long it takes you to get through check-in, security etc (unless you're flying business or first class or have sufficient FF status you can use the fast track security). The biggest issue causing delays is the check in and bag drop process. If you don't have baggage to check and you can check in online then you can head straight for security. Once through it's simply a case of waiting for your gate number.