According to last years' which report, best deals online for family policies are travel-insurance-web.com; simpletravelinsurance.co.uk, travelplan-direct.com and primary1.co.uk. these were the best buys for single trip europe policies (I'm just about to get some myself) and the prices quoted were around £20 for the family (last year).
you might also be able to find the which report online, I don't know.
We just bought through Tesco ... got quotes from Post Office also. An annual policy for worldwide travel through Tesco was £110 inc cover for my son's nut allergy. Through the PO it was £130 or so PLUS another £143 to cover his allergy!!!!!!