GB, we had to take out separate insurance for our daughter when she travelled alone last year. She needs to be with either myself or my husband to be covered by our family insurance.
On another note, was checking for travel insurance for my son and his girlfriend the other night (they are coming to Calis with us for a fortnight in the summer). Moneysupermarket.com - single trip for two weeks for both of them started from only £12.20! I was amazed it was so low and the cover was adequate, including everything they could possibly need.