I can't actually think of many examples where that applies in practice. Pendolinos, perhaps. Shorter IEP vehicle-lengths for midland mainline, possibly - but even then those lengths are only what's been normal up until the IEPs appeared. Can't immediately think of anything else.
Taking Rock rail as the example of "new entrant with sticky leases" they've got:
717s: non-standard, anything else would require end doors and wouldn't represent any capacity gain
810s: non-standard AT300, too powerful on diesel (read, expensive to run) for most other IC TOCs, and too short compared to other AT300s. Can't really be cascaded to other franchises (XC) without building a whole load more of them (which is possibly more of a win for Rock), nor replaced on EMR for any real gain by an EMU compared to just removing the engines
701s: Full fleet of 200m (or 100m) units, no capacity benefits in replacing them as they're again maximised out on infrastructure constraints effectively locking them in at SWR. No chance of returning part of the fleet and replacing with new either (WMR style) as they're a) one ROSCO b) standard
IEP(W): Mandated for use by GW franchise and successors for 30 odd years!
745: No demand elsewhere for a 240m 25kV semi-IC spec EMU (nobody else can take it), and also the 'goldilocks' situation of just the right amount of units on GA (together with the bimodes) to make their replacement highly unlikely - there'd be no capacity improvements and you'd end up with a less standard fleet
755: goldilocks'ed in with the 745s, but would realistically be quite attractive to just about any operator in the UK
West Coast AT300s: Relatively standard, especially the 805s although here at least there's always going to be a market for the ICWC diesel routes and a Hitachi bi-mode is as good as locked in on that until the next generation of intercity trains is introduced (30 odd years away), HS2 won't be operating them. 807s probably the least-sticky of all their fleets
Some are stickier than others, the 717s for example are absolutely going nowhere, whilst other fleets are a bit less sticky but are unlikely to leave that franchise because any replacement (whilst technically possible) would present no benefit over the current fleet. Not impossible, but highly unlikely