So based on feedback I received (thank-you) about not renting a car at Heathrow and driving to Bath right away I have decided to instead rent a car in either Salisbury or Bath after getting there by train/bus from Heathrow. But I just can't decide which option I prefer, so I hope you'll chime in.
Option 1: Head to Bath from Heathrow (arrive mid-morning), check in, Roman Baths or tea, 7:00 town walk.
Friday - get car, sunrise Stonehenge tour, Salisbury Cathedral, Old Sarum, Avebury and West Kennet, back to Bath and 8:00 walk?
Sat. - Glastonbury and Wells, afternoon tea, 8:00 walk?
Sun - head to Conwy via Tinturn Abbey
Option 2: Head to Salisbury from Heathrow, check in and get car, Cathedral
Friday - sunrise Stonehenge tour, Old Sarum, Avebury and West Kennet, drive to Bath and check in (I have a B&B picked out that is out of the city center but still within walking distance and has private parking for its' guests) if time Roman Baths or afternoon tea
Sat- Roman Baths, Glastonbury, Wells and evensong, back to Bath, 8:00 walk
Sun - Head to Conwy via Tinturn Abbey
I don't know if it is better to spend the 3 nights all in Bath and have to drive out to Stonehenge etc.., or if it is worth just the one night in Salisbury to be able to start close to Stonehenge for that day. It looks like I would have to rent from Europcar because I need to drop it in York and that is the only company that is in both cities. Thanks for all your help!