Here are the current rules:
Before departing any country to head to the UK:
All travellers to the UK aged 12 and over have to show proof of a negative test, even if they are fully vaccinated.
This can be a PCR or a lateral flow test, and must be taken in the two days before departure for the UK.
You’ll need to find a private test provider for this – free NHS tests are not allowed.
The test can be taken either in the country where you start your journey, or in another country en route to the UK.
If you test positive, you must follow the rules in the country you’re in, and not travel to the UK.
In addition, all travellers coming to the UK must fill in and submit an online passenger locator form no more than 48 hours before arriving – even if they are just passing through the UK.