The high was recorded at Kew Gardens in south-west London on Tuesday afternoon.
The hottest-ever March temperature of 25.6C was recorded at Mepal, Cambridgeshire, in 1968.
Met Office forecaster Alex Burkill said it was "exceptionally high for the time of year".
Much of England and Wales have been bathed in sunshine but in Scotland a yellow rain warning is in place in some parts.
People enjoyed the change in rules in England, which allow groups of up to six to meet outdoors, including at this central London park
A temperature of 24.2C was recorded in St James' Park, central London
Here people enjoy Bournemouth beach on England's south coast
At Kew Gardens, the mercury hit 24.5C making it the second warmest March day on record
In some areas temperatures climbed from -1.6C at 07:00 BST to higher than 20C by 13:00 BST
At Boscombe beach, Dorset, the fair weather was a chance for members of a local scooter club to dust off their helmets
While these paddle boarders decided it was warm enough to take to the sea
Punters took to the River Cam in Cambridge
Staff from St Thomas' hospital, on the south bank of the Thames, enjoyed a bit of sunshine
And anglers were out on Boscombe Pier
There was sunshine on Brighton beach but the weather is less good in some parts, with a yellow rain warning in place for parts of Scotland