As the UAE is no longer on the UK 'red list' (and assuming you have not been in a red list country in the 10 days before you arrive in the UK), the requirements are:

Book a '2-day' test (to be taken in the first 2 days after arrival in the UK). This currently is a PCR test.
Complete Passenger Locator Form before travel. (the booking details for the 2-day test are required on the PLF

No PCR test required before leaving the UAE for the UK. But you will need to show your UAE 'vaccination certificate'.

All information available on: https://www.gov.uk/guidance/countrie...of-vaccination
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