It's looking increasingly likely that there will be a Thundery Breadown between 25th - 27th July, as a branch of the Jet Stream dives South in the mid Atlantic towards Portugal before swinging back North towards South West England.
The air over most of the UK for the past 2 weeks has been very stable, but the scenario above will scoop up a lot of warm moist air of Biscay, which could provide a classic Thundery Breadown in SW England like we used to get in the summers of the 1980's
http://www.wetterzentrale.de/topkarten/fsavneur.html
After that, anyones guess?