I had 2 successful E261 claims with BA in the last 14 months. Both for the full Euro600.
The second I though they might have quibbled with. I arrived into LCY from Europe and then needed to go to LGW for a long haul to BDA. It was a through ticket (a journey I do frequently). The flight into LCY was delayed 3 hours (tech) and I missed my connection from LGW, arriving 24 hours late into BDA. I filed a claim and got the Euro 600.
Nah, open and shut 600€.
1 – tech, not extraordinary circumstances, so no defence
2 – the 3 hour delay rendered the minimum connection time impossible
2 – long delay at BDA and EC261 counts the through journey as one trip