Status matches to bmi are covered extensively in this thread in the bmi forum. It also gives the fax number several times.

However, as noted as early as the third post in that thread, bmi do not generally match within *A. Most airlines don't match partners, since a status match is an attempt to get a customer away from a competitor and this is considered bad manners within an alliance. You may be able to justify an exception if, say, you are moving from Germany to the UK, but it will be an exception. If you explain in your initial request why your situation justifies such an exception, your chances will improve. Otherwise, you run the risk of being turned down before you have a chance to explain. It will be harder to persuade them to reverse themselves, once they're on record as having said "no," than to justify a "yes" from the start.