Changing Planes at Heathrow

Pretty much everyone transiting through LHR goes through a security screening before getting to their onward flight, regardless of where their arriving plane parks.

What joe32F said. All passengers connecting at Heathrow have to go through security screening between their flights. It had nothing to do with the size of the planes (we have been on many A320-type planes that docked at a regular gate).

Recently there was a good thread on getting to Scotland ( Edinburgh or Glasgow) that mentioned several airlines that did not connect at Heathrow. I believe Delta/KLM was one’ but there were others as well. Maybe someone can help you find that thread—-it is in the Scotland forum.

My question is why this all had to happen

As others have mentioned, because you always have to go through security when changing planes in the UK. Unless you arrived on a domestic flight. The EU has an agreement on common aviation security standards so that passengers arriving from another EU country, or from a country with equivalent security standards (such as USA or Canada), there is no need to go through security again. But even before they left the EU, the UK decided not to take part in that program and only trust passengers arriving from a UK airport. It has nothing to do with bus gates, and I don't think you where dropped landside as there would have been no way for passengers with London as their destination to go back airside to collect their luggage.

and whether there is anything that can be done to avoid having to go
through security a second time

Yes, don't book a flight that includes a connection in the UK.

To this day, I just do not want to pass through Heathrow at all (for
multiple reasons)

You're not alone, that is one of the reasons I avoid Heathrow for connecting flights.

and fear I may have to go on a planned trip to Scotland through
Heathrow since American is not going to be flying directly from PHL to
Edinburgh for a while (because of a shortage of 787s) and may fly us
over to London first in another "puny" plane not qualified for a gate.

There are other options, such as Aer Lingus via Dublin or KLM via Amsterdam. And if you are willing to travel to Washington or Newark you will have more options that do not involve Heathrow.

I have never heard an Airbus 320 described as a "puny" plane. All flights are assigned a gate.

It is rare that aircraft are deplaned away from a gate at Heathrow. More than likely, there must have been a delay with other flights which caused your flight to deplane away from the gate. It has nothing to do with the size of your plane.

I've been through Heathrow more times than I remember and the only time I had to go to a plane by bus was when my departing flight's plane had technical problems at the gate and it was faster to bus us to the plane than to replace aircraft.

Everyone departing from Heathrow must go through security again.

If you decide to go through Dublin, give yourself extra time as you will go through US immigration and customs there as well as security.

My experience has been the opposite of Frank’s. Of all my trips to the UK, all from the Baltimore/Washington area, only one time did the plane pulled up to a gate at Heathrow. We’ve always parked on the tarmac and were bussed to the terminal Pulling up directly to the terminal would 20-30 minutes. Our worst experience was waiting 45 minutes on the plane for the busses to show up almost causing us to miss a connection flight.

You will not fly Philadelphia to London via American on a"puny" plane, AA uses A330s or 787s for this flight. Or sell you tickets for BA, using 777s.