The BA website does advise on the 'Tiers and Benefits' page that the flights and tier points must be collected in a year:
https://www.britishairways.com/conte...s-and-benefits
When you click further into the 'About Tier Points' it expands on what is meant by this, in particular that you have a membership year in which to collect tier points in order to achieve relevant status tiers.
https://www.britishairways.com/conte...ut-tier-points