That's pessimistic, in my view.
1. Typical Leaf SOH decline is about 5% in the first year and 1 to 2% for each year thereafter. There is partly determined by usage and partly by age: even if you did exceptionally low mileage you would not escape most of the decline, and there are some very high mileage Leafs around that haven't suffered much more than average. 5% annual loss for any year other than the first would be exceptionally high for UK Leafs.
2. There's some evidence that, when certain conditions are met, V2X usage actually reduces battery decline, as shown in a University of Warwick study. To simplify, the idea is that there is an optimum zone for SOC -- neither too high nor too low -- where battery decline is minimised, and some V2X usage patterns increase the amount of time the battery spends in this 'Goldilocks' zone.
That said, this is a trial exactly because there are many unknowns, and the exact extent of battery depreciation is one of them. The quid pro quo is that, in exchange for taking a punt, you get a V2H charger (and type 2 charge point) at about half the normal retail price.