Service bus operations are at their absolute minimum on a Sunday. Stagecoach, First, Arriva and Go Ahead will have vehicles sat around in depots all around the country. Accessible coaches perhaps not, but certainly accessible buses.
London in particular will have a couple of thousand vehicles sat around doing nothing on a Sunday (the total fleet is apparently nearly 9,000), and driving them up to Manchester would only take four to five hours. And bus driver wages in London aren't high, so if you offered drivers quad or five times time and two nights in a reasonable hotel while they do it you're going to get plenty of takers.
Every man has his price. If the price being offered doesn't result in anyone signing up, increase it until it does. Northern are saving a fortune not operating the trains - given the difference between bus and rail wages the bus drivers could be being offered quad time or even more - are they?
On a Sunday there are likely about 5,000 or more urban buses not being used across the country. You just need to be willing to pay enough to the drivers to get them out.