A southerly airflow will bring conditions similar to those in southwestern France and southern Spain on Monday and Tuesday.
Temperatures are expected to approach the record figure set at Mepal, Cambridgeshire, in 1968.
London is more likely to experience such unseasonal March conditions, as well as areas just north of the capital.
The heat will be relatively widespread, however, with temperatures reaching the early 20s Celsius in many parts of England and Wales.
And although some parts of the north will be humid, temperatures there will also be above average.
This will be particularly the case on Monday for the eastern parts of Scotland, where temperatures could locally rise into the upper teens.
Average maximum temperatures for the end of March in the UK and Ireland are around 12 ° C (53 ° F) in the south and 8 ° C (46 ° F) in the north.
Easter weekend looks much colder, however, as a significantly cooler air mass is expected to move south by midweek.
While the holiday weekend looks pretty good and settled thanks to the high pressure, some places will experience winter showers.
This coming weekend will also be disappointing for some, with increasingly winter showers.
There will be sleet and snow on Friday, mainly over the northern and western heights of the UK and Ireland, but overnight light accumulations can also occur at low levels.
About 1cm to 3cm is possible at altitudes of 150-200m in parts of Scotland, Ireland, Northern Ireland, North West England and Wales. There will also be a risk of ice, as well as a generalized freeze.
On Saturday, the showers will subside throughout the morning, leading to a mostly dry and bright day, before clouds and rain spread to the northern and western areas.