We have had a family annual policy with Direct Travel Insurance http://www.direct-travel.co.uk for several years and they cover children living at home up to about age 23, I think, even when travelling separately from the parents. Have a look! Hope this helps. They offer good cover too, often recommended in newspaper travel articles.
See http://www.direct-travel.co.uk/faq/can-children-be-insured-if-they-are-travelling-on-their-own.aspx