the rivvy part is all entirely doubletrack and or quiet road wiht a bit through the woods that will be dry anyway.
I see nothing there that requires a MTB to complete and i will definitely be on the CX bike for this ride [ 50% at least is basically road which also applies to the entire ride if not more].
Quite an odd choice of route tbh
Most of it would be doable on a road bike with road tyres. Not often you can say that about an off road route – NB I dont know the bit from Manc to Bolton that well though I have ridden it but again it does not require a MTB IMHO/IME