We really ought to just introduce a new policy that all rail replacement services must be low floor buses. Not just PSVAR compliant but also low floor. There are hundreds of low floor bus operators with hundreds of low floor bus vehicles in all parts of the country so there is really no excuse for sometimes using coaches.
Even the wheelchair accessible coaches are still nowhere near as accessible as a low floor bus. The lifts on some models can be very unreliable and often not work. They often require seats to be taken out which can often only be done at depots. Other types of disabled people and elderly people can find it very hard to walk up the steep steps in to the coach. They are not a good choice for rail replacement.
Almost all the issues that wheelchair users and other disabled people have with rail replacement seem to occur on coaches. So just mandating low floor buses would make it much better. Far more accessible for everyone.
A number of issues with this blanket proposal:
1. Speed - low floor buses generally have a much lower maximum speed than a coach so for long distance travel it will significantly slow the journey. In a previous employment I used to do quite a bit of rail replacement and my employers had a fleet of semi-coach spec Olympians that could comfortably hit 60mph and on Three Bridges to Brighton replacements the London drivers could leave 10-mins before me but I would still overtake them only about half-way down the dual carriageway.
2. Motorways - Most buses are not designed for sustained high speed running so there are reliability issues with running lots of urban spec buses on fast & long runs.
3. Luggage - Standard low floor buses have very limited luggage capacity and for the Intercity routes there are a lot of travellers carrying significant amounts of luggage.
4. Distance - A lot of intercity closures cover quite long distances, this means that vehicles are running on EU rules requiring tachos. Most buses, and their drivers, work on domestic hours so don't have them.
5. Availability - Particularly for Saturdays, as well as the big holiday closures, most buses are still required for normal services so there is a limited supply whilst a lot of coach operators have significant school commitments which means they have spare vehicles & drivers at weekends and holidays.
6. Comfort - Again for long distance flows & closures a base spec bus will not offer the required quality (including possibly toilets).
I'm sure there are couple of other issues and compromises that I have missed or forgotten but it does give a basis of the issues.
Of course having a low-floor bus doesn't really help wheelchair accessibility if you don't have a pavement to load from then the low-floor and ramp are useless and a lift is actually more useful (when I was driving rail replacement around half the station pickups were just in the car park without a pavement though that may have changed now low-floors are more universal as it is over 15 years since I did that sort of thing regularly).
The issue was largely created by the rail industry, someone (or multiple someones) got it into their head that PSVAR didn't apply to rail replacement work and so no one specified such vehicles and so none of their regular contractors bought them as the rail operators wouldn't pay the extra cost of using them - you need to remember that any vehicle built after 2005 and used on rail replacement should have been wheelchair accessible from the start. It is only just before the final deadline when all coaches needed to be wheelchair accessible that it was pointed out that they should be using wheelchair accessible vehicles which caused the crisis we are still dealing with - not enough vehicles, not enough experience for drivers and managers and compromised retrofits that are less optimum in usage than a purpose built vehicle.