We use
http://www.hiscox.co.uk/
They insure firearms as part of the contents insurance without having to know all the details. They are also covered when you take them out of the house.
Quote from their site, "With Hiscox home insurance there's no need to list valuables, such as precious metals, jewellery, watches or guns, of less than £15,000 in value, as long as the total amount insured for valuables covers them. There's also no need to list fine art and antiques, as we cover them as standard as long as they the total amount insured for contents covers them too."
We have home and contents with them at around £75. Not sure if that is expensive but when we tried to cover our guns while we were with Virgin they told us to find someone else. We found that many companies were not interested in our business when we mentioned guns.