Thank you Dave Noble for your input.
Your reasoning also the way how BA is putting forward its argument.
Again, I fail to see the clear language in EC261/2004.
I may derive "sth." from this definition:
Definitions
(h) "final destination" means the destination on the ticket presented at the check-in counter or, in the case of directly connecting flights, the destination of the last flight; alternative connecting flights available shall not be taken into account if the original planned arrival time is respected;
The e-ticket was presented at the check-in counter at PHL with destination of MUC. The passenger never went to a check-in counter at LHR.
The destination of the last flight was MUC.
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After having paid EUR 250 and accepting the full 100% compensation for LHR-MUC in writing, do you think BA has grounds to invoke the 50% rerouting clause?
I have another example of AB paying EUR 600 for this case without a court fight and a lawyer:
JFK-HEL-MUC on one ticket with an online connection:
JFK-HEL on AY was punctual. HEL-MUC on AB was cancelled. Passenger was rebooked one day later on LH.
I do not think AB would have paid, if there was doubt whether JFK-MUC or HEL-MUC is setting the distance.