Update for entering (returning to) uk: Boris Johnson has confirmed that fully vaccinated travellers will no longer have to present a negative coronavirus test (PCR or lateral flow) before being allowed to embark on their journey to England.
TLDR : Answer Summary
As of 4am on January 7, 2022, the UK has lifted the requirement for fully vaccinated travelers to present a negative coronavirus test before embarking on their journey to England. Instead of a PCR test upon arrival, vaccinated travelers are now allowed to take a cheaper lateral flow test within two days of entering the UK, without needing to quarantine while waiting for the results. However, those transiting through other countries should check with their airline for specific testing requirements, as rules may vary.