We've just got back from camping in the Lakes. I have a 2-3 season bag which was fine. Probably a 2 season one would have been fine too and I could have worn more clothes (slept in leggings and a long sleeve t-shirt, no socks). I also bought a fleece sleeping bag liner from Tesco in case it was cold and various fleece blankets from ikea at £1 a go in case it was cold but didn't need either.
My sleeping bag is a square one too as I don't like the restriction of the mummy ones. They are, apparently, warmer.
So basically I've waffled and may or may not have helped! I think what I am trying to say is if you have a lighterweight sleeping bag there are lots of options if it ends up being a bit colder than you thought
I have a 3 season bag, probably more than you need for August camping, but there's nothing worse than being chilly at night, and it's versatile enough that I can use it in spring/autumn as well. Never done proper winter camping! It's a fairly cheap/basic, mummy-shaped one from Blacks, keeps me warm enough anyway. I like the mummy shape as you can draw in the 'hood' around your head to keep a bit warmer if necessary.
we got two of these www.gooutdoors.co.uk/outwell-camper-brown-stripe-p147579 they were half price when we got then a few weeks ago, we bought left and right zips so they can zip together. i really feeel the cold as i cant regulate my body temp very well and this was perfect for me, i fact its the best nights sleep i have had for months. they dont crinkle and are soft inside and square at the bottom and they unzip to make a blanket