From the Guide:
There are often questions raised in FlyerTalk as to whether BAEC will assist those who narrowly miss the tier point thresholds at the end of the membership year. There is evidence of flexibility in terms of extending the membership year by a week or two, so if you are in need of some flexibility call your local service centre after the relevant flights have posted.
https://www.flyertalk.com/forum/30593331-post4.html
See also the experiences in these threads:
British Airways Tier Point Expiry and 2 week extension.
BA Two Week Extension
Tier Point Expiry
Tier Points Grace Period - How Long to 'kick in'?
Collection year extension period.