10l should be plenty big enough for 50 miles, I've got a 15l which is big enough to be totally self-sufficient (all food) plus full w/proofs. Like Elspeth my wproofs get plenty of use up here. When I did saunders mountain marathon in July in Lakes, was mega hot by day but freezing at night - I had to put my waterproof trousers on inside my sleeping bag as I was too cold at night
Serves me right for taking a tescos £10 sleeping bag to cut down on weight!!!
I grew up in and lived in Surrey until I moved here last year ... sometimes August is super-hot, sometiimes it a wash-out; equally I was running in shorts and t-shirt and catching the rays on the moray coastal trail just a month ago, today it's 1 degree and we are getting snow downpours in bbetween the sun and my washing was frozen on the line. Be prepared to be flexible with your kit, check the forecast right before race day and all will be well whatever you get wherever you are
Great running LLB, I've got my long run tmrw - hoping to hit 22+ if I can. Fingers crossed weather isn't too atrocious!