Bath to Heathrow

I took the National Express Bus from Bath to Heathrow. I pre purchased my ticket, but I think you could buy it through the app on the day of travel if you wanted. I found it a very efficient way to make the transfer and it involves minimal luggage handling.

Train every time for me as I would prefer to not be on a coach so long.

The coach to Heathrow from Bath is about £20 this far out. It does go up if you leave it up to £40 ish.

The train is £23 if booked now - that Advance fare disappears about three weeks before travel and then is more like £40. Then you need to get from Paddington to Heathrow - Express is £5.50 if booked 90 days in advance, otherwise Elizabeth line for £13.30 or a one change slower Underground trip for £5.60.

The possibility of being delayed "for hours" is minimal.

The coach drivers have sat nav, and also have 24 hour contact with control. Control are very efficient at re-routing coaches when there is an incident. I've seen that first hand several times.
Not only for RTA's but one time when the M1 was closed due to a suspected bomb on an overbridge.
Cars were delayed for hours. We were sent round it and incurred minimal delay.
Of course the coach could get trapped between junctions, the same as you could driving, or being driven in a taxi/uber/private car service.

It's always best to pre-book the coach, especially if travelling on a Sunday afternoon. The morning is always quieter.

Coach is just more straightforward, no luggage handling, no changes.

The train, depending on route taken, is either one or two changes, and cope with your own luggage. Same rules as the coach on pre-booking.

The big question is what time your flight is. If anytime before early afternoon I would say spend the last evening at Heathrow- particularly if delays are going to worry you. On a 3 hour check in a 1pm flight is 10am arrival (please do not cut that tighter), which would mean a c 7am departure from Bath- on train or coach.
If it's a late afternoon or evening flight you have more time to play with.

It has been a while since I made this very journey, on the train (to Richmond?) and onto Heathrow via the bus, and it was on a Sunday - but it was also during peak summer travel in August - and I bought the tickets well in advance and the train and bus were full / sold out on the day of travel. Whatever you do, I suggest you buy the tickets as far in advance as possible.

Would I do it that way again? Probably not, at least not doing the travel via train and bus to catch my homeward bound plane the same day from Heathrow, in case there is a problem or delay or missed connection. That was the last time I did something like that, and since then I put myself into position much closer to the departing airport the day before. In other words, I would end that trip in London for a few days rather than the other way around, leaving London to the end of the trip (any trip to England includes at least a few days in London for me, YMMV). That is how I construct all my itineraries now, give myself that cushion, makes for a fairly stress free getaway day.

There is no fixed 90 day limit on the Heathrow Express £5.50 fare. That is old, out of date, information. There is a quota of those fares, which can sell out at any time, thus £16.50 is already the lowest fare for all Sundays in October, November and December. 31 December is currently the next available Sunday £5.50 fare.

You're not flying out the same day you're traveling to Heathrow, are you?

I flew out the same day I traveled from Bath to Heathrow. No reason to automatically tell someone not to do this. I took an early morning bus and had a mid-afternoon flight. I was there 3.5 hours early.