Charlie,
The surge of hot weather from the south, the so-called Spanish Plume, is pretty rare, and computer models lack quantities of data for accurate predictions, but a ridge of high pressure fills in from the SW, with a low over N.France slowly moving north Friday night. The speed of movement of the low will be critical, and I don't think they can predict accurately.
That said, we're in for some pretty unusual conditions Friday/Saturday, with 30°C likely max, extremely high humidity, fog at times, heavy, and slow-moving torrential thundery downpours with associated strong winds.
To give an example of the weird conditions, and see what the West Country is getting in the next 48 hours:
Lyme Regis to Lands End including the Isles of Scilly
Strong winds are forecast
For coastal areas up to 12 miles offshore from 0600 UTC Thu 17 Jul until 0600 UTC Fri 18 Jul
24 hour forecast: Wind Northeast 3 or 4, veering east 5 to 7, increasing gale 8 for a time then veering south 4 or 5 later.
Sea State Slight becoming moderate, occasionally rough later in west.
Weather Thundery showers later.
Visibility Moderate or good, occasionally poor at first.
Outlook for the following 24 hours:
Wind South, becoming variable later, 3 or 4.
Sea State Moderate becoming slight.
Weather Thundery showers later.
Visibility Moderate or good, occasionally poor later.
I'm watching this carefully too!
Mike
PS Several years ago I was caught in the shipping lanes in a thunderstorm. The torrential rain flattened the sea, the radar was useless as it couldn't penetrate more than quarter of a mile, and ships were then invisible. The gusts when they came were 30 knots. I felt like a blindfolded hedgehog walking across the M25.
PPS Recommended approach for lightning is put all portable electronic devices (phones, car keys, mobile VHF etc) into an oven in case of a nearby lightning strike. Even if turned off, electrical induction from the massive voltages can destroy circuits. The oven acts as a Faraday cage, and devices inside will survive.