Arrive @ 7a - Bus leaves @ 8:20 - too close??

You will see much more countryside from the train but the bus is convenient. Also note train tickets are much more expensive if bought on the day of travel.

Does the coach company provide flexible tickets if you opt to go by road?

If you have checked luggage, you need at least two hours to clear immigration. Most of the trans-Atlantic flights arrive about the same time so the immigration lines get quite long. If you only have carry-on, you might be able to do this if the bus stop isn't too far from immigration. Heathrow is huge - expect a long walk.

You are going through Immigration not customs!

Heathrow is notorious for Flight delays ( landing and departing ).

IMHO trying to cutting it too close but your trip your choice.

How much are you willing to risk? You might catch that bus, but you might not. Would you be upset to lose the fare you paid in advance?

Also what terminal do you arrive at? How far from arrival terminal to the bus station you plan to leave from?

The new egates have made going through immigration much quicker, but collecting luggage can take awhile.

Personally I'd book the next direct coach at 1225 which should be about the same fare as the 0820. Then you have a buffer in case of any flight delays before you even arrive at LHR.

You do not indicate where you are coming from and when the trip is. Weather at either end ( or both) could affect your arrival time.

Having to retrieve checked bags is the wild card. Somebody has to have the last bag off the plane.

The other issue already mentioned is where do you get on the bus in relation to bag claim area of whichever terminal is your arrival point.

To me it seems you would need everything to work like the proverbial Swiss watch to pull it off.

A plan that requires perfection at every step along the way is not what gives me a warm fuzzy feeling.

You can add a "change & go" option for £5 which allows you to change your coach for 12 hours before or after your scheduled departure. It worked out great for us as we arrived just 5 minutes before an earlier coach than ours was departing and they changed our tickets quickly enough. We arrived in Bath several hours earlier due to the flexible ticket option so it was well worth it. If we had arrived late we could have changed to another coach in the subsequent 12 hours.

I thought the coach scenery was pleasant enough and especially so as we slowly went through a small Cotswolds town. Also, it was so comfortable that my travel partner easily napped while I watched the cows and sheep.

In which terminal do you arrive?

Will you have checked luggage?

Where are you arriving from?

Check the bus website to see whether you can pay an extra fee at the time of purchase so your ticket is flexible, allowing you to take a later bus if you need to. At one time, National Express allowed that; I think the fee was 5 GBP in 2017.

The new egates have made going through immigration much quicker, but collecting luggage can take awhile.

The 13 year old won't be able to use the e-gates so the family will have to visit a manned booth which is where the long delays could occur.

The amount of walking involved, especially if arriving at one of the satellites, can be quite substantial and time consuming. Baggage collection is usually quite quick particularly at T5.

08:20 would be optimistic in light of the potential time spent at immigration and the distance required in transit.

Is there a particular reason for going straight to Bath upon arrival other than because Rick suggests it? I've never understood the recommendation.

You only have to be 12 to use the egates, so your family should be able to use them. Last time I flew into the UK, I got through the egates quickly, but waited more than 45 minutes for my checked bag.

It is possible (but risky) that you could make it, so I’d definitely pay the extra 5 GBP to get the ”change and go” flexible option mentioned by previous posters.

National Express shows at T2/3 - does that mean in between??

It means the bus stop serves both Terminals 2 and 3. It is probably slightly closer to T2 than T3. Google maps shows that it should take you 7 minutes to walk the 0.3 miles from T3 to the bus stop. It is at street level directly above the Underground station but you get to it by walking through tunnels with travelators from T3.

Look for signs that say "Central bus station"; if you can't see any follow the signs for "Underground". You will end up in a section with a big arrivals/departures board. Check which stop your bus leaves from. National Express buses should be the white ones.

It is marginally quicker to do it on the surface but for anyone not familiar follow the signed route through the tunnels.