Originally Posted by MSPeconomist

This would be a connection from T3 to T2 at LHR. I'm not sure you can do it without entering the UK, but most passports won't need a transit visa for a same day LHR international to LHR Schengen connection, especially if your passport doesn't need a (Schengen) visa for Germany.

AMS should work well for this or forget DL miles and just take a LH TATL flight to MUC (or connect in FRA if it's more convenient).