LHR to Bath via National Express - Timing

I did this in 2016. The tickets are not expensive and if there was a price difference, I didn't notice it. I waited to buy mine until I'd done everything else I needed to do.

I had to wait awhile before it was time to get on my bus, but there was a coffee shop for breakfast right in the bus terminal building and toilets nearby.

I would not buy tickets in advance. The time between your predicted arrival and your preferred departure is too short. I arrived about 8 days earlier than you will. I don't think there were more than 5 people on the bus, so I'd be very surprised if the 10:20 sold out.

I'm not sure what you mean by a direct route. We stayed on the same bus the whole time, but we went through lots of little towns. I thoroughly enjoyed that. The arrival bus terminal in Bath is adjacent to the train station.

If you plan to return to London or LHR from Bath, be sure to ask for a return (round trip) ticket if you want to take the bus back. There was a man in front of me in the ticket line who hadn't done that and spent a lot of time chewing out the agent because she hadn't suggested it. It was going to cost him a bit more to get a one-way back than the RT would've cost. It appeared that he hadn't said anything to her about returning to LHR. I guess she was supposed to be a mind reader.

Tempest in a teacup, but it was amusing.

Going from memory, I think in 2017 I paid a bit extra (maybe 5 pounds or so) for flexibility on a National Express ticket from Luton Airport to London. I don't remember the details, but I think I would have been able to take a somewhat later bus if my arrival had been delayed. Perhaps that add-on would allow you to take a chance on booking the earlier bus.

Thanks for the advisement! I am worried that 7:50am will be too quick, but we're not checking any luggage so just waiting at customs. It would be tough to get out at 7:30 and then wait until 10:20 to board the next bus. The flexible ticket may be a good option, will check into it.

I bought the extra 'insurance ' to be able to change tickets if arriving early or late. If you do this, be sure to print out receipt. I booked an early departure but flight was delayed. Cranky woman at counter said I had to buy a new ticket. Showed receipt, which she read thoroughly and then grudgingly gave me the new time. Everyone else was lovely. I would buy ahead so if you are close to 7:50, you go right to bus without the delay of the purchase desk.

"but we're not checking any luggage so just waiting at customs."

Actually where you'll spend the time is waiting at Immigration for a border official to scan your passport and ask you a couple of questions. If you land when several other international flights have landed you'll have a longer line. If you hit a seam where you are between big flights then you may get thru quickly. We were delayed last summer after the Delta flight crew told passengers that only one landing card per family was needed so my brother and SIL filled out one between them, then had to go back to the end of the line and fill out a separate one for SIL. IF they are still using landing cards (had read somewhere that they were going to stop doing this and that it was all electronic) be sure to fill out one for each of you.

You'll also have a walk from your arrival terminal to the National Express bus terminal which is between Terminals 2 and 3. It took us about 15 minutes last year to do this and I knew where we were going as I had done it the year before, lol.

I landed on British Air in Terminal 5 but did not need to go thru customs as I was flying from Bucharest. I found the National Express counter and was advised I had to go to Terminal 2/3 and I could take the Train to get there. It would have been a 2 hour wait so I decided to buy the bus transfer to the Reading train station and there I bought my train ticket. I arrived much earlier in Bath so the added expense was worth it.
On my return from Bath, I decided to do the same trip via train and bus. When I arrived at LHR airport, I figured out how to get to the central bus station and take the local bus to my airport hotel.

If you are landing in Terminal 3 with American, it should be straight forward to go through customs and find the ticket counter and buy your ticket the same day. There are nice coffee shops and my favorite Pret Marche restaurant with wonderful salads, sandwiches, ect for take away or eating there. I always try to stop there to have good food for my journey!

Hope this helps.

I underline what Pam wrote. It is the border security and immigration process that can cause long line-ups. Customs is merely a matter of collecting any checked baggage and walking through a door marked Nothing to Declare. Spot checks are possible, but rare. Also, There's no controlling a line-up at the coach ticket sales point either. Even ticket machines can be busy. The "queue" is as much a part of the English experience as cricket and tea.
Anyhow, it is the uncertainties of long-distance air travel that are most likely to disrupt the best laid plans.
PS: I have heard airline personnel, who know the ropes, still use the word "customs" in a confusing manner. Ah, well....

Pay the extra £5 for the change and go option, then aim to catch the 7:50am, but if you don't get it, just hop on the 10:20 - it's a very small price to pay for the extra peace of mind.
From the National Express website:
For just £5 each way, our Change & Go Add On lets you be completely flexible with which coach you board to and from the airport:
Board any available coach 12 hours before or after your original departure time
Amend your ticket to a new departure date - free of charge
Refund on your ticket up to 24 hours before departure
So if your flight is delayed, then there's no need to panic; you can just hop on the next available coach.
Simply add the 'Change & Go' option to your ticket in the Add Extras section of the booking process.

Thanks to all for the advice. We did take the Change & Go option and booked at 7:50. Fortunately customs had a like just half the size of the full capacity, and we also landed well on time. We ended up at the bus center by 7:10 and were able to get coffee at Cafe Nero while we waited for it to arrive.