Today could be the mildest December 22 since records began.
The Greater London area is the most likely to beat the record of 16.1C set at Hoylake, Merseyside, in 1910.
Both the Met Office and MeteoGroup said today could see that record broken.
Even before 8am this morning, MeteoGroup had recorded temperatures of 14.3C at Heathrow Airport and 15C at London City Airport.
A spokeswoman said: "We've not got a great deal of rising of temperatures to do to match the record and I think we could see that matched today."
The Met Office said it was "finely balanced", but sunshine in afternoon could put the heat on Hoylake's 105-year record.
According to one of the group's forecasters: "It's going to be a bit of a push to break it, but it's a bit finely balanced.
"We will see a few bright spells towards the South East, and if any brightness develops at the right point, there's a chance we could get to 16C at least.
"Generally speaking there's been quite a bit of cloud, but if the sun peeps through the clouds in early afternoon, it could just push the temperature up to 16C or something like that."
The outlook was less sunny for the North, with a wet and windy start to be be succeeded by showers moving in from the north-west.
There are warnings for rain today in south-west Scotland, north-west England and west Wales.
Scotland, too, could see higher temperatures than average. Aberdeen could hit 12 or 13C this morning and then drop off, with the afternoon temperature across Scotland more likely to be around 8 or 9C.
Predicting mixed weather over the days leading up to Christmas, the forecaster said: "It's going to stay very unsettled, with a band of rain moving from the west over the next few days.
"It could just be cooling off across the hills of Scotland at times, giving a bit of snow, but it will be the highest ground in Scotland, so if there's a white Christmas anywhere it will just be the mountains of Scotland.
"The two main features of the weather are the rain today and then on Wednesday night there's quite a deep area of low pressure passing by to the north-west of Scotland, which could give some very strong winds on Wednesday night coming into Christmas Eve morning.
"No sign of any significant cold weather for some time yet."