my understanding is (and i may be wrong) as long as you've had continuous cover you can switch insurers each year but you have to be 100% sure your new insurer gives retrospective cover. from my uni days i remember 5 years being a realistic date to backdate stuff. unlike car insurance claims are normally made moths if not years after the event and its the insurer your with at the date of the claim that pays up. this has been a particular problem for me as i once had £1m cover only because it was cheep and sounded good. ever since then i have to maintain this riduculous level of cover for effectively house extension jobs. 

Hi Eliza,
this is a good question. I'm no expert, but Ltd Co status may have the advantage of providing an extra line of protection if for some reason the insurance company decided not to pay out. Also, as far as I know (please correct me here if I am wrong) the PII only covers you whilst you are paying the premiums, so if in a few years time you decide not to keep doing private work, you will still need to keep paying the premiums until the time when you think any more claims are unlikely. The Ltd company status might be useful at this point.
Best wishes
Mike