https://www.schiphol.nl/en/messages/coronavirus-update
Are you travelling to Schiphol from abroad and are you 13 years or older? Then, with effect from 00:01 on 29 December 2020, you are required to show your airline a declaration that you tested negative for COVID-19. The declaration must be brought with you throughout your entire journey. This can also be done digitally. Your negative test declaration may be checked when you arrive at Schiphol. This obligation applies to all passengers flying to Schiphol, transfers included. This is also mandatory for Dutch citizens.
This will disrupt plans for many travelers. I have a flight booked for 30 December and was surprised not to hear from KLM regarding this.
The requirement is for all countries, not just the UK.