Mid-week and no school holidays. At mid-day, assuming you spend some time at the LHR arrivals lounge, the traffic should be relatively fast flowing unless there is an accident on the motorway.

Stonehenge is out west and is not on the way to LHR. Personally, I am not sure if it is worth the effort unless you are a very strong interest. Many find visiting the site relatively disappointing especially when you cannot get up close. Security around the area steps up around that time since the summer solstice is on the 21st.


If you do not have a lot of luggage, I would second the suggestion to visit central London. You have several transport options, train or road (taxi or bus) and there is more to see and do.