Yeah, it's important to take recommendations for things like sleeping bags, mats, jackets etc with a hefty pinch of salt. Some people are impervious to cold, while others are perpetually shivering.
I wouldn't go near a sleeping bag with a comfort rating much above 5C for a British Summer. Indeed, I use a three-season bag all year round. Cooling down a sleeping bag is just a matter of zips, making one warmer requires the foresight to bring some silk liners at a bare minimum.
Unfortunately, cheap means synthetic, and synthetic means either cold or bulky...