After our RS tour I'm thinking of taking the train from Edinburgh to London. What is the cost and can we stop off in Manchester?
Cost: could be anything between £252 (First class, travel any train) and £20 (fixed LNER train, early booking).
If you want to go via Manchester then two separate tickets will almost certainly be the cheapest way of doing it
As Marco says, the cost depends on what type of ticket and when you buy it.
London to Edinburgh trains don't pass through Manchester.
Buy tickets from the train operator https://www.lner.co.uk/
Manchester is not on the direct route from London to Edinburgh. If you want to stop off there you may have to change stations from Queen Street to Central in Glasgow (not far, but awkward if you have lots of luggage).
TransPennine Express have direct services between Edinburgh and Manchester Piccadilly.