The ORR have indicated all rail replacement transport whether emergency or planned is subject to PSVAR.There's no 35 mile limit for the buses, quite apart from which much rail replacement work is less anyway. You're probably getting confused with the 50km limit and the domestic hours issues for drivers.
As far as I know there are exceptions for PSVAR vehicles in an emergency situation.
There is plenty of rail replacement work over 30/35 miles and I have always been advised by the road transport section coaches must be used for this work as buses are not allowed. This may be a company standard but similar advice has been received from local bus firms when contacted as a last resort. Maybe it’s permitted with a change of driver en-route (not very practical).
One class of vehicle which is permitted to run without being PSVAR compliant is a minibus with 22 seats or fewer, but they can’t exactly shift large numbers.