The typical inter city journey in Britain is between 100 and 150 miles. London - Birmingham is about 110 miles; Birmingham - Manchester about 90 miles; Manchester - Leeds 40 miles; Leeds - Newcastle about 90 miles; Newcastle - Edinburgh about 105 miles
100 miles at an average start-to-stop speed of 100 mph takes 1 hour. At 140 mph the journey time is 43 minutes - a saving of 17 minutes. What would people do with the 17 minutes if they did not spend it on the train, and what is the cost of saving this time, since energy consumption is more than double?
And that is only the station to station time. Realistically, the door to door journey time is more likely to be 1 hour 28 minutes instead of 1 hour 45 minutes, a useful amount admittedly, but it could equally well be saved by local transport improvements or better connectivity, which would be of benefit for everyone making local journeys.
That is not the entire story either. High speed rail services are not walk-on services, passengers will have to arrive longer in advance in order to be certain of not missing their bookings, so the journey could end up taking longer!
The sort of journey many people might want to make fairly often is Brighton to Oxford. This takes about three hours by crowded and uncomfortable train, including at least two changes. Thirty years ago there were two through trains a day taking 2 hours 35 minutes.
High speed rail is of limited value unless there is good connectivity, and good connectivity would transform the appeal of conventional train services without further vast expenditure on high speed railways.
Locally (East Sussex), I could list a whole string of rail improvements that could be implemented quickly and would be of real benefit over a significant area, achieving improvements in connectivity. No doubt anyone familiar with their patch could do the same. This would yield a long list of worthwhile projects, which high speed rail will force off the agenda.