We purchased tickets to London and now have decided to spend some time in Edinburgh first. We would like to know how difficult it is to go from terminal to terminal so we can determine how much time we need to make a transfer in order to purchase tickets from London to Edinburgh.
Allow no less then 2 hours to transfer between terminals. Connecting passengers have to go through a complete security screening, and immigration check. In addition you may have to stand in line at a transfer desk to get your boarding pass from Heathrow to Edinburgh.
This is as complete answer as I can give. It’s by the manual and it assumes you have cleared passport control, baggage reclaim and customs. You may get lucky and sail through those three hurdles in half an hour but you have to be pessimistic and assume it will be crowded, so allow a full hour minimum.
The connection between Terminal 1 and Terminal 2 is a 15 minute walk.
The connection between terminal 3 and terminals 1 and 2 or terminal 4 is a shuttle bus. The length of wait for the bus and the journey time depends on how crowded things are but again the pessimistic viewpoint says 40 minutes. So allow 40 minutes for transport unless you are transferring between T1 and T2.
Allow 40 minutes minimum to check in at the connecting terminal. As you are on a domestic flight from LHR to EDI the process is a little quicker that if you are making an international flight.
The official website states 2 hours. It has never taken me that long - and as the poster above said several are 15 mins walk from each other. That said, I would be uncomfortable having less time between flights just in case there were queues. It is not at all not at all difficult in my view and is very well signposted etc, it just takes a bit of time but in some respects less than some huge airports where there are not separate terminals. Dubai T1 for example advises people to allow 40 mins to get to the gate.
In the future, it's often better to buy connecting tickets on the same airline or alliance because they will book the connections and have to care of things if you miss a connection. And your luggage will be checked straight through which is a huge advantage in Heathrow where it can be hike to Terminal 1, where the domestic flights depart from.
For separate flights, be aware that many airlines - especially the discount airlines - require that you be checked in at least 40 minutes prior to departure. And they can be VERY strict - doesn't matter whether you were in the line or not, you've got to have your boarding passes at the 40 minute mark. So I would plan on being at the checkin at least 1hr - 1.5 hours in advance if it is Easy Jet etc.
You might want to allow some extra time in case your flight from the US is late. We had about a 4 hour time window for our connection last year and ended up with only 2 hours because our flight was late getting into Heathrow. The walk between terminals was not bad but once we got to the other terminal, the lines for check unbelievable. We had our boarding passes but had to check our bags (when no liquids at all were allowed). We found some airline personnel directing people and they took us to the front of the line after we told them our Virgin Atlantic transatlantic flight was late. I don't think they would have done so if we had just not allowed enough time....