Hi,
This is from the EC terms and conditions on ba.com
"12.9. Mileage and Tier Points which are not tracked automatically at the time of travel may be credited later at British Airways' discretion. Members may claim Mileage and Tier Points after completion of their travel either online at ba.com or if required by submitting the retained segment of the boarding pass and passenger receipts
provided that the Member was enrolled in the Executive Club at the time of travel and the claim is made, within 6 months of the date of travel or the relevant transaction. A new Member may claim Mileage and Tier Points for eligible British Airways flights taken up to 3 months prior to the commencement of their Membership."
Existing member - 6 months
New member - 3 months
It looks like you won't be able to claim for those flights
K