There are two issues to be determined:

1. For whatever reason, OP has still not answered the one ticket vs. two ticket question. He has three times answered the "ticket" question by responding about PNR's and Codes. Neither of the latter are relevant. So, still need to know if he was on a single ticket.

2. If on one ticket, the sole question is whether the 19 minute delay on LHR-HEL was an "extraordinary circumstance." If it was, OP gets nothing. If he was not, he is entitled to EUR 600. It is a binary issue but he is stuck with the problem that a 19 minute gate, air traffic or weather delay is inconsequential and, if asserted by BA, will be hard to disprove.

Claims agencies may be a good test. They only get paid if they collect, so they do not accept cases which are not a certainty. If OP is willing to toss EUR 150 - 200 of a total of 600, that is the route to go. But, I would be more inclined to file a clear and concise claim directly which is 2-3 short declarative sentences and focuses on the only relevant facts: 1. One ticket, 2. Delayed more than 4 hours at the final ticketed destination, and 3. No extraordinary circumstance.