In reply to Edward, as I remember it the last time anybody suggested we should build maglev high-speed trains in the UK was George Osborne about ten years ago. But Osborne would only be making such suggestions as part of David Cameron's "Vote Blue Go Green" campaign, where the Conservative opposition tried to appear to be even more Green-leaning than the Labour government of the time.
The main reason maglev is so little used is that its costs are prohibitive, about five times the cost of a conventional railway system according to this 2006 BBC news article:
Wiki has good article on maglev.
However I had thought the French had investigated this. I seemed to remember seeing a section of abandoned track in a documentary - probably by Jonathan Meades - but it was perhaps instead the Aerotrain.