I must admit, I was pretty surprised when I read this Flyertalk thread, but it seems that it is correct. You still receive 1,000 BMI Diamond Club miles – instantly – when you sign up for BMI Diamond Club (link here) and use promotional code 1JA1KD. I tried it yesterday and it works! However, I expect BA to close this down pretty quickly.
(If there is already something in the Voucher Code box when you click on the link above, delete it before adding the 1JA1KD code.)
Since July 3rd, you have been able to transfer BMI Diamond Club miles to British Airways Executive Club Avios, using the BAEC ‘Combine My Avios’ function in Manage My Account. You cannot transfer BMI miles into an avios.com or Iberia Plus account directly, but this is easily worked around by transferring them first to BAEC and then to avios.com or Iberia.
So … if you or anyone in your family does NOT have a BMI Diamond Club account but does have an Avios account, here are 1,000 easy miles. Sign them up to BMI Diamond Club and then transfer the 1,000 mile bonus out. You must have a matching date-of-birth in your BA and BMI accounts in order to do the transfer.
Note that you cannot do this for children because kids can only join Diamond Club via a family account, and the sign-up form for that does not allow you to input a promo code.