Those that are coming in are bringing home British citizens who were stranded overseas (in some form of lockdown) and essential workers.
On 17th April it was reported in Times:
"At least 15,000 people a day are flying into the UK without checks on their medical condition, the health secretary admitted yesterday.
Matt Hancock said that the equivalent of 105,000 passengers a week were entering the UK from countries including those with serious coronavirus outbreaks such as the US, China, Spain and Italy. Criticism has mounted over the failure to impose health checks or a compulsory period in quarantine for people arriving at UK airports."
www.thetimes.co.uk/article/coronavirus-flights-bring-15-000-people-a-day-to-uk-without-screening-bn255vtjs
vast majority of arrivals will get onto public transport.
I'd be interested to see that these 105,000 passengers a week are all UK citizens or essential workers? I didn't realise we had any restrictions on people entering UK.