The BMI gates are quite far, as you may know, from the main check point at LHR.
From TXL, you will most likely have it far easier as the security and check in is right before the gate, and if you are taking a taxi you can usually be dropped right in front of the appropriate gate area. (If you haven't been to TXL before look at a diagram of the airport and you will see that it has a rather unusual layout)