That's the reason why the UK desperately needs High Speed rail, rising property prices
I think the opposite.
The reason house prises in the South East are so high is purely because of the difference in supply and demand, what with being so close to London and the employment opportunities that presents. If, however, you could get from the Midlands to London in the same time as from the 'suburbs' then this will surely open up a wider market and consequently reduce demand nearer London as people look further afield for cheaper housing. The affect of this will be to reduce house prices. I guess the local prices further North and West will go up but I would say the overall impact will be downwards.
For me, the more High Speed routes the better. Imagine stations at Bristol, Swindon, Birmingham, Nottingham, Derby, Sheffield, York, Doncaster and Norwich all being with 60 mins of London. You could almost pick from 30% of the country to live in rather than the 10% in and around London. Bring it on.