Update 2022-01-25: According to recent information published by the authorities, the current requirement for vaccinated individuals to get tested within two days of arrival in England or Scotland will be dropped with effect from 11 February. As of that same date, unvaccinated travellers will also no longer be required to self-isolate or to undergo another COVID-19 test on day 8 of their stay. More information is available via the links below.
UK Government, Scottish Government