Thanks JB for all the great info. Agreed re: staying near hotel...I think getting everyone in one spot should be first priority and then once everyone is at the hotel (overnight), then we can travel together. Appreciate the reminder about delayed flights as one option was to try to get to Southampton Sunday evening - so we'll scratch off that option. We're traveling from 3 different US cities.
For airport hotels, I'm currently considering Hyatt Place (The Grove, Bath Rd, Harmondsworth, ~£103/dbl occupancy), but am open to other suggestions (especially if I can get a quad occupancy hotel room that's reasonably priced).
I'll request quotes from your list - again thank you!
What do you all think about a Stonehenge (or Winchester, or ??) stopover enroute to Southampton? I know it will add to the cost, but if we don't do any sight-seeing in London, it might be nice to have a stop.