Countryside is free. Stay at one of the b&bs at:
http://www.farmstayuk.co.uk - average price only about �25 and the best value you can get, staying in listed buildings, old farms on the coast or inland - wherever you fancy, and get some fresh air into you for a couple of days.
Or camp - cost of getting there and your scran. Thats a cheap break.
As for where - what do you want to spend on travel? City stays will be more pricey, but also loook at the YHA - loads at good prices and often in a central location. Edinburgh has a new hostel due to open in August which is when the festival is in full swing, so loads of street theatre etc and some at really good prices for the shows in the day too.
If you want to get away completely - look at the Ryan Air site - flights from 19p - and book a youth hostel in Berlin - �10 a night on the International YHA site.