It's a combination of things. Most of the well paid jobs have migrated to London and the South-East, causing very large pressures on house prices in that region - but at the same time, lack of land, government policies hostile to social housing and the available land in London mostly being used to build speculative housing aimed at overseas buyers, have all caused significant upward pressure on prices in and around the capital. That has in turn led to a 'spillover' effect in which house prices have risen significantly across the Home Counties and most parts of Southern England.
In the rest of the country, the pattern is similar apart from the mass developments of luxury blocks for speculators. The general picture is one of insufficient new build, lack of development land, a government blockade on new social housing and hostile policies towards the release of land in country or exurban areas outside the existing suburbs. This has pushed prices up in all of the large cities across England apart from the far North. In Scotland and Wales, apart from a few hotspots, (Cardiff, Central Glasgow and Edinburgh), prices have largely stagnated.
The migration of all well paid jobs to the SE corner of the country has also created a large deficit of affordabl housing for the younger end of the population, who cannot afford the escalated prices, even if they are doing good jobs in London. There is a vicious circle of excessive concentration of work in London and the SE combined with house prices rising even more rapidly.
Brexit and other pressures appear to be slightly reducing pressures in London, but property will still remain unaffordable to most as jobs collapse at the same rate due to Brexit.