Parts of the country will reach 28C by the afternoon, also surpassing top European holiday destinations such as St Tropez, Marbella and Santorini, with conditions dry and sunny across England and Wales.
The Met Office has said it will start a consistent run of high temperatures over the next few days, meaning Britons could bask in an official heatwave.
London, the South East and East are expected to see highs of 28C and 26C on Friday afternoon, while the South West is slightly cooler at 25C.
The capital will cool slightly, by one or two degrees, over the weekend before picking back up to a high of 29C on Monday and Tuesday and even a sweltering 30C on Wednesday, the Met Office predicts.
This means London will be in a heatwave.
The Met Office defines a heatwave as when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold.
The threshold varies by UK county. For London, it is 28C.
Heatwaves are becoming more “likely” due to climate change.
London reached 31C in the hottest day of the year on June 17, when the UK sizzled during a heatwave three weeks ago.
In August 2003, the UK experienced heatwave conditions lasting 10 days and resulting in 2,000 deaths, the Met Office said. A record maximum temperature of 38.5C was recorded at Faversham in Kent.
Meanwhile, the UK Health Security Agency (UKHSA) issued a level 2 heat-health alert warning ahead of the heatwave.
The alert is in place from 9am on July 11 until 9am on July 15, and covers the East of England, South East and London regions.
The heat-health alert scale is designed to help healthcare workers manage through periods of extreme temperatures and has four levels.
Level 2 is reached when there is a 60 per cent risk that temperature thresholds will be reached in one or more regions on at least two consecutive days.
Greg Dewhurst, forecaster for the Met Office, said: “Over the course of this week, much of next week, temperatures are going to be above average and very warm locally, hot at times.”
He added: “Over the next few days, primarily the highest temperatures will be across southern and eastern parts of the UK. But I think as we go through the weekend and into next week, the heat is likely to be across pretty much the UK.”