We are travelling to France for an extended stay which is over 90 days (~100). Our motor insurance only covers for 90 days out of UK and have told us they cannot extend this period. Could I cancel this policy after 30 days abroad and set up a new policy with another company to start while we are away and reset the 90 day period? Any other solutions?
In my opinion it would be smarter on your part to subscribe an ad hoc insurance in your home country. I imagine it would be easier to file a claim from the UK with a UK company rather than having to do this over the phone with a French company. To this purpose you should look for European car insurance, which extends the basic insurance cover - the minimum needed to drive legally in Europe - which should be included by default in your preexisting UK instance contract.
A quick search on a comparison website might come in handy here. A random query on Google revealed this company which seems to provide a policy with annual foreign use included.