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

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