Problem is, there doesn't really seem to be anything preventing a UK based BMI Gold to sign up to the EC with an address in one of the eligible countries, before sending in their BMI Gold card and becoming an instant euro-cheat Gold.

As BA are relying on a clear photocopy of a BMI card, and obviously won't be being told by BMI where members are registered, there's no way they can know just from a BMI card where that person's Diamond Club account was held. Rather open to abuse.