Cancel the policies and take out new insurance.

Garage the cars, SORN them and take out new insurance when you return, although this will mean neither will be covered for anything happening to them.

Can't you renew remotely when the time comes, or will you be somewhere without communication ('phone/internet)?

Suggest put in details for 29 days hence and when you find an insurer you fancy dealing with, ring them up and explain the situation and dates required, chances are they'll work something out for you.

Don't forget Direct Line and some others (NFU if i recall) are not on comparison sites but DL for one have their own get a quote website and are very good to deal with on the phone.

If your own insurer can't see a way around this issue it suggests to me best look elsewhere.

Some insurers (not sure which though) do operate in a 60-day window. It might be worth (if this site doesn't have such details) going to Money Saving Expert and seeing if they do, as it's likely it'll be update in this regard more often, as insurance advice is one of its major aspects.