This week we've had a battle between the dry, warm air from the continent in the south and the cooler, moister air from the Atlantic in the west. There's been a frontal system sitting out over the Irish Sea/just into West Wales for the last few days, but as the winds have been light, it's not been moving (hence the really warm temperatures hanging around in the east). However, the system finally moved eastwards overnight and during today.... so temperatures are cooling off now, with cooler air coming in from the west behind the frontal system.
Up here in Shropshire, we're looking at maximum temperatures of about 19 degrees or so over the weekend with sunshine and showers (but more/heavier showers on Sunday). Temperatures looking to still 21-23 degrees in the south (mainly southeast - London area) for the next few days but around 17-19 degrees further north.