The rainy season is any time between January and December, usually increasing in likelihood in April (hence the phrase "April Showers"). In Scotland it's worse!!!
In the Scottish Highlands you have an increased likelihood of blustery weather in late October, and a high chance of Gales in Jan/Feb.
Temperatures are often at their highest in July/Aug, peaking at around 80*. Roughly translated; bring a raincoat and a sweater or three...
My last-but-one car had aircon, but it broke through lack of use!!!
Honestly, you shouldn't come to Britain for the weather. Why do you think it's so green?
The air quality is pretty good, though, especially with all that lovely vegetation.