Any traffic jam between Bath and Heathrow on Sunday Afternoon?

nope as long as, no accidents interrupting!

The route from Bath to Heathrow is almost entirely on the M4, and doesn't pass through any city centres, so it's relatively OK. M4 is at the moment not subject to any major roadworks other than a bit of resurfacing etc. around Reading (J10-11). Sunday afternoon is usually not too bad traffic-wise, so you should be fine. (That said, all it takes is one accident, and the whole thing comes to a grinding halt...)

The traffic oround the M25 expecially around Heathrow is busy every day.
I would guess that the coach companies are well aware of this and have built in an allowance into their timetable.

I always err on the side of caution and plan on getting to the airports well ahead of time,if there is a coach that should get you there an hour or so earlier then take it.
It is a lot more stressful sitting in the airport for a few hours as opposed to sitting in a traffic jam-panicing

The M4 is always very busy and is often slowed down by heavy traffic particularly between Reading and Heathrow. Accidents, roadworks or extreme weather can create long jams, so allow lots of extra time