The problem with UK rail, and particularly Merseyrail, is the lack of standardisation dating from how the system was built/managed over 150+ years.
Leasing is fine when stock (like B737s or A320s) are standard issue across hundreds of airlines, so they can flex their fleet sizes and deployment as the business changes.
UK rail is still stuck in a time-warp where a multitude of different standards apply, making many fleets tied to the routes they operate for decades at a time.
So there is much less scope for efficient leasing business models.
But "GBR" is still a long way short of the B737/A320 go-anywhere-almost-at-the-drop-of-a-hat standardisation model.
Merseytravel has bought its new fleet for a 40-year stint, like the 50x being retired (now approaching 45 years).
If they have too many/too few trains, nobody is going to help out.
Last time, BR sent a portion of its Southern fleet to Merseyside to save money - that won't be happening again.
Regional and intercity fleets have rather more standardisation, so can benefit more from efficient leasing deals.