I've heard great things about www.simplybusiness.co.uk. They do some amazing things creating resources for small businesses.
I really like the fact all documents are available online as well which means for certain businesses you can actually show your documents to clients etc.
I think as a starting point if you have premises you'll need public liability insurance as well as buildings and content cover. You may also need to look at employers liability if you will have staff operating from the premises.
I use AXA UK but will admit that I didn't do all that much research into it - because I had so many other things to think about at the time that I just wanted it sorting out, no matter how much it would cost me to do so. So I would be happy to hear about what you decided on, and hear how it's working for you, etc. Make sure that everything's covered that you need. Nothing's worse than finding a cheaper insurance, then finding that whatever you need they won't pay out for.