Hi Cindy
We have lived in England and stayed there most years, so have had a lot of experience with LHR and traffic on the roads.
You will need to be at LHR at least 3 hours before your flight. Security can be very slow. Motorways in England have very heavy traffic and often there are significant delays. Suggest that a flight at 12 noon is not a good idea.
Of course you can be lucky if all the ducks are in a row but our experience is that it’s not worth taking the chance.
