Train from Bath to Heathrow Airport

Thanks for asking!

5 hours.

2 hours to Heathrow- arrive by 10am for a 1pm flight, add an hour as contingency for unforeseen events= a 7.13 departure from Bath on a weekday.
On a Sunday the first train at 7.58am is overly tight.

About 5 hours total.. Since you need to arrive at Heathrow by 10:00, you need to catch a train a bit before 8:00. Expect a change of train at London Paddington.
https://www.nationalrail.co.uk/

Or you could look at Flixbus, with a direct route to LHR in 2hrs 20min. I think there's one at 07:30. https://www.flixbus.ca/bus/united-kingdom

ETA: given that I'm rather risk averse, I might suggest heading to a Heathrow hotel the evening before your departure. Then you wouldn't have to leave the hotel till 9:30 or a bit later.

I second CJean's advice to check on bus services. With advice from this forum, we took the direct bus from Bath to LHR and it was direct, easy, and inexpensive. Prior to this trip I had been reluctant to choose bus transportation over taking the train (I don't know why :) I am a convert now in that I will at least compare the two options, before automatically thinking the train would be easiest.

Thanks, everyone! Great advice. I'll check both trains and buses to see what works best for us. Love this forum!