I have a camelbak ultra 10 which is not a cheap backpack but it fits me well and has lots of space to carry stuff. It has a cord so you can compress your stuff if the bag is not entirely full. It also has a waterproof pocket at the front in which I can put my phone but it is not big enough if you have a big phone ( anything over the size of an iphone5s would be a challenge to fit in,imo).
Take a phone in a ziplock bag with you if you are running alone in case you get into Trouble. Maybe also have a look if there is an app which lets you contact emergency services without phone reception. There is one available in Australia as it can be dangerous to go in the bush, but I'm not sure if there is one for England.