I think I got that backwards. United for Terminal 4 and Thai for Terminal 3 - so for each terminal what would be a good place for breakfast? (Oh and not the English breakfast please!)

By airside I guess you mean after security and landside before you pass thru security?

I think you've got it right. Airside would be AFTER security and landside would be BEFORE going thru.

Airside is the side of security with the airplanes, i.e.within the secure area. Landside is the area outside of security.

When you arrive at T4 on United, you will progress through an arrivals area which doesn't have access to the departure area. There are no shops or restaurants in the arrival area. You will eventually reach a point where you will be required to make a decision whether you transfer to T3 internally, airside, or exit airside and transfer landside.

If you decide to remain airside, you will be directed eventually to an inter-terminal bus, which is the bus you refer to above. This bus will transport you securely across the surface of the airport to the departure area of T3. In this departure area you will find shops and restaurants. You won't have access to shops or restaurants at T4 because you will be confined to the arrivals area.

If you decide to exit T4 and transfer to T3 landside, or outside of security, you will be required to go through Immigration before you exit the terminal (T4). You will most probably find yourself standing in the line for immigration for a minimum of 30 minutes, unless you have an EU passport. The last two times I went through Immigration at LHR, in February and April of this year, the wait was closer to an hour.

When transferring landside, you will take the local portion of the Heathrow Express, a train which goes from terminal to terminal as well as to and from Paddington station. There is no charge for riding between terminals but as flanner said, it's relatively infrequent so you may end up waiting 15 minutes or so if you've just missed a departure.

When you return to T3 for your departure after your breakfast 'outside', you will be required to go through the security checkpoint before you are allowed back in airside, with all the annoyances and delays that entails.

In addition, you should take into account that the terminals at LHR are large, very large, and it's not unusual to find you have a 15-20 minute walk to your gate.

So...yes, you have almost four hours between flights but if you decide to go in search of a great breakfast outside of the secure area, or landslide, you may find yourselves rushing to make your next flight.

In contrast, four hours will give you sufficient time if you stay airside and have breakfast in T3. I agree with flanner that this is your best choice.

Here is a link to eating places in T3. Please note they are listed according to whether they are landslide (at the top of the list) or airside. Sorry I can't give you any recommendations - most of my Heathrow time is spent at T5 now.

By the way, I'm flying to Bangkok in September this year and again in January 2013 (new grand-baby) and will transit through Heathrow both times. I was going to say that maybe our paths will cross, but on second thought, I guess not because I'll be flying in and out of T5. I see BKK is one of your favorite places, Bill - mine too!

Whoops! Forgot the link to eating places in T3!

http://heathrow.airport-information.co.uk/restaurants/terminal-3.html