The high speed service in Kent is a complete joke, because only the top 10% can afford to use it, whilst the rest of the South Eastern commuters subsidise it.
As for the rest of the shower of s***e in Kent, the local lines to London are probably slower than in Charles Dickens times. When I worked in the City, I could cycle the 30 miles home quicker than the train journey (2 trains 1hr 30m).
A decent high speed service would use different, bespoke tracks - no crossings - and limited stops. In other countries (France for instance) they cover greater distances, so can maintain higher speeds, and the whole network is tailored to fit in with them. The high speed services in other countries are also affordable.