Actually I have found that the Concourse A line has been slowing as of late, now that people know about it.

Here is the best thing to do, when you get out of Int'l arrivals you will be on Level 5 of the terminal. Straight ahead there will be a security checkpoint (North Security). If you have a small bag, you can use the express lane there (line on right). A backpack or laptop will put you in the other line (longer line on left). If the line does not look to bad, go there. If it is only a couple of weaves then it is not too bad.

Otherwise straight ahead and on the right there is an esculator that will take you to Level 6 and the security point at Concourse A. The esculator will actually take you back above where you just came out. If you go through at Concourse A, after security you will walk across the bridge, when you get on the other side walk towards the British Airways Club and take the elevator down to the train (It says next to the button). This will save 3-4 minutes. When you get down there you will want to take the train that is on the right hand side, to Concourse B.

FYI, based on the time of your arrival people will be heading to Concourse A for the BA(777) and LH(A340) flights. F9 is on that concourse and they have blocks around 5p and 8p, if I remember correctly.

For the flight back you will just need to take the train from Concourse B to Concourse A, and check-in with the LH staff. This is all within security.