80% of Britain's population live within one-third of the country's land area. If you draw a line around and area that includes Southampton, Bristol, Birmingham, Liverpool, Manchester, Leeds, Sheffield, London and Eastbourne, you have included 80% of Brtain's population. You have excluded most parts of the country with significant populations, the only substantial conurbations being Edinburgh/Glasgow and Tyne and Wear. This means that most inter city journeys are less than about 200km. High speed rail produces little worthwhile time savings at those sort of distances.
Within the most densely developed regions, a structure has built up since 1950 which can most concisely be described as sprawl, and it is difficult to service by any form of public transport. Until this changes, the car will be the preferred method of passenger transport. The main changes will be relatively marginal and achieve through the development of park and ride schemes, tramways, and congestion charging. Their main benefit will be to make the urban environment safer and pleasanter, especially for people when on foot. This is not a negligible gain, but it is not going to save the planet, if indeed the planet needs to be, or can be, saved.