Please be aware, you must check in one hour before you flight in Heathrow.. we were in line for our 12:15 flight at 10:45 am. By the time we got to the front to check in, it was 11:30. We were not allowed to check in and had to find another flight home. I wouldn’t chance it.
Also, if traffic is heavy, it could take an hour and a half or more to get to the airport from South Hampton.