Today, nowhere officially. But during the last 18 months or so there have been matches into both Skyteam and Star Alliance. They are usually posted on here.

BMI will also match your BA status immediately if you have a BMI flight booked (not necessarily flown ...). However, *A Silver is less use than BA Silver because you only get into the lounges of the group that issues your card, in this case BMI - so no Lufthansa etc lounge access. You really need to be BA Gold to benefit from a BMI status match, as *A Gold gets you into all the lounges.