This needs a comparison to the diesel bill.
While, electricity is going up in general. The UK rail lines are running diesel-electric trains that can run from electric lines when available. Over the last 10 years, a huge move to increase the electric lines within the UK has happened. Much of it completed recently.
Also add to this, the new government is planning to move those diesel-electric over to battery trains that can charge plus run on the electrified lines. Then use battery on the non-electrified sections.
So a significant % of the cost increase will be intentional and the result of reduced diesel use.