HS2 was justified by valuing business users time at £47/ph (2010 prices). That is where the vast majority of the B in the BCR comes from.
The C represents the stonking cash loss it makes, with revenue not coming close to the construction costs even over an appraisal period of decades.
Is the £47/hour the amount that the staff are paid or the amount that they are charged out to clients? They are very different things and before you consider if this is high or low then you need to understand the difference.
Clearly being paid £47/hour would be a very highly paid member of staff (circa £92,000 per year). However even that's over estimating the cost as there's other remuneration package costs (pension, etc.).
As such if you're looking at pay then it's far too high.
However businesses have many other costs associated with staff and so when they charge clients for their time it's done so in a rate which covers these costs. These include office space, training time, other non chargeable tasks, etc.
Now £47/hour is a very low charge out rate for staff (in engineering that's comparable to a graduate's charge rate and they're unlikely to be going on trains to meetings without other staff, however charge rates of £100/hour or more are fairly common, especially amongst those attending meetings). As such it's likely that a figure of around 50% had been used, as although you can get on with some work you are not going to be able to do as much as you could do sat at your desk. Not least that you'll not be able to work on your phone as fast as you can type at your computer, or its going to take you some time to set up your laptop before you can start and you'll need to pack it away before your stop. Even then you're not going to be able to work as fast, and you're not going to be able to have the same connection speeds to your data as you have in the office.
Also if the member of staff is a manager it's harder to manage people when you're not near them.
Therefore £47/hour isn't excessive and is probably about right for the time saving for business travel if it's being based on charge out time.