I lived overseas for 24 years:
Singapore, 85 miles north of the Equator. Always hot - but still wore cardigans in the freezing cold cinemas with air conditioning set at arctic temperatures!
Switzerland (between Zurich and Lucerne). The Summer weather was often hot, but evenings could be much cooler. Cardigans easy to carry around instead of a more cumbersome jacket.
New Jersey/New York City; Extremely hot, humid Summer weather. (Just checked the weather for the July 4th weekend - 30C to 33C sunny with possible thunderstorms. Cardigans rarely needed, maybe a very thin one for air conditioned cinemas.
Moved back to England (south west coast).
What is Summer???? So far this year every evening has been very cool (even a little cold at times). Even during the daytime it’s not very warm if not out in direct sun - it can feel chilly if a large cloud blocks the sunlight or when the wind is blowing. I’ve hardly worn my shorts or short sleeved tops and T shirts. Dresses and skirts haven’t seen the light of day yet! Unfortunately I just feel cold so easily.
Soooo - the British weather is so unpredictable and even if the sun does come out it can turn cloudy and wet a few hours later. No lovely warm, sultry evenings. I can’t cope without taking a cardigan along with me to take on or off as the temperatures fluctuate.
Sweatshirts/fleeces/hoodies/packable down puffer jackets/sweaters are fine with jeans, shorts or leggings but not with a pretty dress or skirt. Jackets are sometimes not lightweight enough and either too casual or formal.
I’ve got a few cotton Summer cardigans (mostly Fat Face/White Stuff/Next and a favourite old Mantaray from Debenhams). I can shove them in a large handbag or backpack (with often an umbrella lol) and pull them out when the temperature drops, or even just drape over my arm. I’m never giving them up in this climate!