By that same logic, there was no improvement in building the Channel Tunnel or HS1 because it was possible to fly between London and Paris in approximately the same time as the train takes today.
Most of the time savings from HSR come from the complete lack of fuss at either end, as passengers only need to arrive a few minutes before departure and they can be out on the street mere seconds after arriving at their destination. Even the slickest business traveller going through the slickest airport isn't going to be anywhere close to this, especially when you then have the fact that most airports are relatively distant from not only central business areas but also where it is easy for a large number of people to reach. If you live in Paisley it won't take you long to get to Glasgow Airport, but if you live almost anywhere else in Greater Glasgow there will be a not-inconsiderable journey which will also be challenging to do by public transport. On the other hand, the HSR terminals are all located in central hubs of public transport, so it would be just as easy to get from Paisley to Glasgow Central as it would be from anywhere else in Greater Glasgow (especially after a north-south heavy rail tunnel is built).
The cost of the scheme will depend upon a variety of factors but I cannot see it being that much more difficult than any of the other sections, such as the one through South Yorkshire. In particular, the number of people who would live or work near the route is going to be tiny, so the amount of expensive compensation works that will be required will be small. The southern sections might be easier to deliver from a pure engineering perspective but once you include all the extra compensation works, e.g. extra and longer tunnels and cuttings, needed for political reasons it's not really that much more impossible. If I had gone around on this forum saying that the four political parties with the largest number of prospective seats for 2015 (Lab, Lib, Con & SNP) all heavily support the choice to build the most expensive (and beneficial) HSR scheme before 2009 I would have been laughed at, but here we are today with Phase 1 in the committee stage and all that.