A British Airways informa:
Dear Travel Partner,
Changes following Brexit mean that from 1 October 2021, your customers will not be able to use an EU, EEA or Swiss national ID card to enter the UK unless they are exempt. Please check the latest UK Government advice for the full details to make sure your customers are able to travel. You can also read answers to frequently asked questions here.
Thank you for your understanding. We look forward to welcoming your customers on board soon.
Regards, British Airways