Fundamentally, choices made by the government in the '60s regarding transport development meant:
1) a large increase in fast roads into the centres of many cities and larger towns, particularly the midlands and the north
2) a similar experience was planned into London, e.g. the M4 and even M1 but that proved too expensive and would result in the unnecessary dumping of large volumes of traffic in fully developed residential and light industrial areas
3) the presence of multiple major rail routes into central London, including a distributor network of metro railways, (i.e. LT Underground, East, West and North London Lines and a web of Southern metro links) that rendered most trunk routes beyond the North (A406) and South (A205) circular roads unnecessary.
Like it or not, London is vastly different to any collection of settlements elsewhere in the UK, and it isn't going to change much in the forseeable future.