The more I look at the groups that are available in BC the more I see in Entra that are also not available and I can't work out why. I think it's because they don't have members with BC licences.
I wondered if the group I created isn't showing up because - with GDAP - I'm almost certainly not a M365 Admin in my client's tenant. But then I asked my client (who is an Admin) to create a group and theirs isn't available either.