On 16 November 2021 GOV.UK published a new update on safety rules and regulation for passengers traveling via airport. According to published update,
"Passengers transiting ‘airside’ through England do not have to complete a UK passenger locator form."
Which means from this date passengers who wish to transit via any airport in England, do not require to complete the passenger locator form and hence they do not need to book a Covid test for UK, unless they are planning to leave the airport.
Meanwhile passengers should check their final destination's rules for travel prior to their travel as those rules still applies to them.
Please check UK government website for latest Guidance on Coronavirus (COVID-19): safer air travel for passengers