I think this must be the most absurd argument of the entire campaign:
Like most British libertarians and conservatives, and unlike many Progressives, I actually like Britain. We British have given a great deal to the world, and we have much to be proud of. But it's nonsense to suggest that we can't occasionally benefit from foreign inventions. Indeed, we have a long history of taking the best from around the world, and making it our own.
Now, I'm not sure who first dreamt up AV, and whether it is superior to FPTP is a matter for debate, but I can't think of a worse reason to reject it than "It isn't British".