Insurers have a pretty good idea as to average contents value, based on number of bedrooms. They will want to have details of any individual items of value £200+, especially if they're portable, (jewellery, music systems, PCs, etc.).
A good place to start for a quote is your car insurance people, as they may well give you a discount. But make sure you phone at least two places for quotes.
Think about why you want insurance before you phone round ... if you think burglaries are a real danger, then you probably want a low excess. If you (like us) don't have a lot that's portable and easily nickable, and are more bothered about the thought of the whole lot going up in smoke, then a high excess will save you money on your premium. Some companies give additional optional cover for accidental damage, but think carefully before you take this up, as any claims would affect your next premium, and check the terms carefully as it may not cover what you would think of as 'accidental damage' and some things are covered for accidental damage in a normal policy anyway.
Oh also some policies will give you cover for certain things out of the home, (like mobile phones and cameras) sometimes extra, sometimes not. Think about what you do really want cover for, (i.e. what you would bother making a claim for).
HTH