I very much doubt, that British Midland will comp a LH FT (Silver) - LH and partner SK each hold 20% of BD's shares.
You don't have to give up your miles&more status if you prefer to fly other StarAlliance-airlines (like BD, UA and all others). miles&more even honours every flight with all StarAlliance airlines with status bonus miles (counting as status too) - the other programs (you mention UA) don't.