The cause of the traffic is LHR. The road network is not perfect, but it works.
Sorry but that is just wrong. Traffic from the M1 round to the M23 including the M40 M4 & M3 is all likely to go round the M25 past Heathrow. Then you have the M4 & A4 into and out of London. LHR is just part of a very busy interchange.
And it doesn't work much of the time.