Police were called to a lido in London and beachgoers warned not to overcrowd the seafront as sunseekers in England sought to make the most of the bank holiday weather.
Metropolitan Police officers were called to Ruislip Lido in north London to reports of a large group gathering there.
The Government has urged people to continue to follow social distancing guidelines.
"Please use common sense when it comes to social distancing," the Met's Hillingdon branch tweeted.
Elsewhere, crowds flocked to beaches and parks around the country to enjoy more good weather, a week after the Government relaxed guidance to allow people to visit beauty spots in England.
Hundreds were seen on Bournemouth beach, where the local council had warned people to "think twice" before heading to the seafront after nearby car parks were filled.
"There are other less busy but equally lovely open spaces to enjoy or please #stayhome," the council tweeted.
Large stretches of traffic were also seen at the Yorkshire Dales, while walkers were seen enjoying the grounds of Windsor Castle.
Temperatures had been expected to reach 24C in London, with 21C in Newcastle, 20C in Edinburgh and Belfast and 22C in Southampton.
While new cases and deaths from coronavirus have fallen nationally, a hospital in Somerset was forced to close this weekend due to a high number of Covid-19 patients.
Weston General Hospital, in Weston-super-Mare, stopped accepting new admissions, including into its A&E department from 8am on Monday.
As the hospital shut as a precautionary measure, some residents in Weston reported concern over high numbers of people enjoying a day at the seaside.
Two people died in separate incidents along the Cornwall coastline after warnings RNLI lifeguards would not be able to patrol Britain's beaches.
It is hoped lifeguard patrols will return to 70 beaches, which will be chosen based on risk and popularity, by peak season.