I wonder whether BD would be willing to give you silver immediately if you could subtly hint that BA would be happy to help instead?
Give them a ring and see what they say.
That said, getting BD silver isn't difficult anyway. What is your typical travel profile? If you frequently travel on the same BD route, it may be worth getting a "Individual Routepass" (book of 5 return tickets, valid for 4 months). This saves 15-20% over the full flexible fare, and gives you instant silver status.
So what have M&M done to upset you? I am interested because I have just started using M&M in preference to BD Diamond Club because of the better mileage rates. Maybe the grass is always greener...
By the way, I don't think residents of Germany are able to join Diamond Club - have you got a UK address too?