According to the UK newspaper I read, it has been fairly hot there, up to 98F (36.7C) earlier in July.
I have a lavender plant (L. dentata) in a pot on a sunny windowsill because it isn't likely to survive a winter outside here and I don't want to risk it. It is a far more thirsty plant in a pot than I expected and absolutely hates to get too dry. Ditto for rosemary in a pot, surprisingly. I checked a few articles from lavender growers this morning and it seems this is a known thing with potted lavender (and rosemary) - while they certainly don't want wet feet they both also hate to get too dry.
On another note, having gravel in the bottom of a pot doesn't increase drainage (it actually reduces it- the technical term is a perched water table) but does reduce the amount of potting soil available to the plant. Personally I would replace the gravel with more potting media with same texture throughout the pot. You say the soil was mixed with plenty of grit, did you use garden soil or a soilless potting mix (or maybe John Innes if they still sell that there these days!).
