How much time do we need to get from Bath to Heathrow airport with a flight leaving at 1:00 pm?
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.