Are you suggesting that e.g. American S.L. uses different optimization criteria than British S.L., which explains their differences?

Yes, but not in the way you mean. British Deaf kids learning BSL use the optimization criterion of communicating with the British Deaf community; American Deaf kids learning ASL use the optimization criterion of communicating with the American Deaf community. It's the same reason that French kids learn French and not Tagalog. Sign language isn't different from oral language that way.

The story isn't just about two different optimization criteria. It's about two different fake optimization criteria. We don't actually select which language to learn (or teach to kids) based on features such as those that the ambassadors praise. We select which language to learn (or teach) based on what community we (or our kids) need to communicate with.