The last time the mercury hit this level was back on 19 April in Treknow, Cornwall.
Wednesday’s temperature was also the highest enjoyed in the whole of Scotland for 2019, and the best since 2 September 2018 when it peaked in Lossiemouth.
Forecasters have said the recent warm weather is “fairly unusual” for this time of year in the UK.
During the last eight out of nine years, only about 20% of May days have seen a maximum temperature of 20C (68F) or higher.
But Met Office forecaster Greg Dewhurst said it looked like Wednesday would be the warmest day of the week with more cloud on the way.
He said: “Tomorrow there’ll be a little bit more cloud around generally across the UK.
“It should be mostly dry but there is the risk of one or two showers for western parts of Scotland, perhaps Northern Ireland, but most places will be dry with still some sunny spells.
“We’re probably looking at maximum temperatures tomorrow around about 20C, maybe 21C, so just coming down a little bit.”
Mr Dewhurst said temperatures at the weekend could reach 20C or 21C (70F), and it will not be as sunny.
Sky’s weather producer Chris England said: “Thursday morning will be dry and sunny again for much of the UK, but northwest Scotland can expect a few showers, while the Northern Isles will stay rather grey.
“Southern Britain will be cloudier than recently, with perhaps the odd shower. Ireland meanwhile will be cloudy with rain in the West.
“It will be quite warm generally, but cooler than Wednesday, especially near the east coast.
“Northern Ireland and Donegal will see a few showers during the afternoon, but there will be little change elsewhere.
“Friday will bring cloud and a scattering of showers to much of England, Wales and Ireland. Scotland and Northern Ireland will be fine. It will be cooler for most.”