The cost of such a contingency would probably be far too high.
First you would need to keep ALL the pilots current on the 355 as a type, then you would need a second line of spares maintained [notwithstanding that some will already be in the hangar], maintain a disimilar type, and bring the single tired airframe up to somewhere near the same standard operationally as its cohorts - buying a new Wescam 15 for a start [the new aircraft are using the 355 Wescams].
If - heaven forbid - this were to be the case I suspect that once the chapter on 355 is closed it would be cheaper to wet lease in 355's [or anything] and just go fly with the local police observers to cover the gap. When the 355Ns were down they were 'replaced' by a leased in AS355F2 G-CAMB while things were sorted.
The Met is perhaps more important than some other units because of their location but such a move was reflected recently in Cheshire. They had their BN2 offline for a Wescam fit and they brought in an AS355F2 to cover the gap period. As a result their fixed wing pilot was temporarily sidelined and another brought in. Not ideal but it did resolve a perceived problem and gave the observers a fully operational mount [spare BN2s with sensor fits being rare].