To my mind, there’s only three types of service reductions coming:
1) peak services into the big cities, principally London. These trigger particularly high marginal operational costs (trains that might operate 2 services a day). The difficulty will be picking which services go. There is no doubt that the main suburban line into Waterloo will not need 18tph in future, but which 2-4 trains get removed?
2) ‘competing’ services on the same route, often introduced as ORCATS raids. TPE north of Edinburgh being a good example. You might like to suggest your own candidates. My bugbear is the occasional SWR services to Bristol, or the GWR services to Brighton. Both cause a lot of extra cost for negligible benefit.
3) services in area services of maximum track capacity utilisation, where removing one or two services will have a disproportionately good effect on performance. Castlefield corridor is obvious, Birmingham - Wolves and the Cross City similarly, and countless places in the Home Counties.
Clearly, double money if you get 2 out of three, and Bully’s Special Prize for the full house.