I’ll be storing the contents of our 3 bedroom flat in a couple of weeks. I spoke to the Big Yellow Self Storage, who I believe are a national chain. For a 150sq ft storage unit they quoted me £72 a week – but the first 8 weeks are half price. I’m looking to store for at least 3 months, so that worked out pretty expensive. I had a bit of a look round and found a local guy (unimaginatively trading as The Green Self Storage Company!), who has a warehouse with storage units inside, on his farm, and he gave me a quote of £30 a week, for the same size unit, for as long a period as I like, with access whenever I wanted it.
So I guess my advice would be to shop around, as you may find a local independant doing decent deals.
Edit – sadly for you, nowhere near the Cotwolds, I’m afraid!