More often than now now, it pays to just book well in advance.
Are you looking at mtb/cycling trips or just general stuff?
I’ve done a couple of trips by National Express coach. If you can put up with the long journey time, you can get some real bargains. Standard for me is an overnight from London to Aviemore for £50 return. They’ll take a bike in a bike bag or one of those CTC plastic ones.
Sustrans routes give good touring ideas too. Book an early train to the end of a route and then ride back or book two random locations with somewhere interesting inbetween and ride between them.