My tuppence worth
a) Woodulike is spot on, the devil is in the detail, pay particular attention to the exclusions and the excess's so you know exactly what you're getting.
b) Don't fall for specious special offers like "includes £1m Professional Indemnity (PI) cover", these often mean little. The chance of any self-employed tradesman claiming under PI is so ridiculously small it means nothing.... they might as well throw in free Alien Invasion Insurance for all the good it'll do you. Not saying there's anything at all wrong with such policies, it's just that the "free chocolate teapot" doesn't make them any better either.
c) Don't be too influenced by individual horror or "they're great" stories, I'm not sticking up for anyone here, quite the reverse.... they can all be pants, like everything these days any claim you make will go to a big call centre type place and how well it is handled depends largely on how good the person who picks up your file is. They could be great, they could be pants... you'll never know until you have to claim. Add to this the fact that claims handling can be subcontracted or you might be buying from an agent, so you could go with say "Electricians Ins" only to find that all "Electricians Ins" just write under a binder and/or the claims are handled by say.... AXA, who in fairness to at least some of the thousands of claims staff at AXA... then may well handle it beyond reproach.
So find a policy that covers what you want for a price you want to pay and enjoy the fact that you are buying insurance in probably the best insurance regulated country in the world and that your claim WILL be paid.
Or of course you could try using an old fashioned broker, a qualified professional who can advise you... offer you tailored advice, give you assistance with any claims and you don't pay a penny for it..... of course they're not that easy to find these days now everyone DIY's their insurance on the interweb.