Hi there,
I would certainly look at this as a viable option. See if you can get LH to check you through onto the bmi flight or at the least check your bags through, that way you can come through the transfer facility. Do you desperately need what is in your car or can you take it with you on holiday? A pain I know, but it would speed you up.
You can't do online check in with bmi longhaul, so that option would be no good to you.
LH are in T1 and bmi in T3, so the actual walk is very short between terminals.
PM me if you need any other questions answered WRT bmi or MAN.
Lexx.