Usually your insurance isn't void. Check your documents, mine doesn't say anywhere that it is an insurance requirement to have a current MOT
If you get caught it's a £60 fine, no points
edit: found this
Re no MOT invalidating Insurance; Because of section 148 RTA 1988 as to condition of the vehicle, Insurance Companies cannot invalidate a certificate issued as far as making it a criminal offence i.e. to use without Insurance (S145). This does not not however prevent them from taking action to recover certain costs under the civil law.