But that's not V2L, it's V2G and they are very different. V2L couldn't be used to augment your home supply, but V2G (which is what Octopus and others are trialling) can be. Although similarly named, and using the same base components (the car, it's battery and the input/export port), they are fundamentally different and not interchangeable. There are possibly others, but the only vehicle I know of that currently can support V2G is the Nissan Leaf.
As you say, storage batteries are small and expensive compared to those in an EV - if V2G worked it could be argued that it's cheaper to buy an old Leaf and just use that than smaller, expensive backup battery packs/powerwalls.