The ONCLON fare you're looking for on BA within Europe has this rule text:
END-ON-END
END-ON-END COMBINATIONS PERMITTED WITH BA FARES WITHIN
EUROPE. VALIDATE ALL FARE COMPONENTS. TRAVEL MUST BE VIA
THE POINT OF COMBINATION.
This means that no other carrier may appear on the same ticket with these BA fares. You'll have to buy it as two tickets, or go to a more expensive fare class on BA.