if going to the USA, plenty of medical cover (no NHS there). If you have expensive cameras, good valuables cover - though when I lost mine I claimed under my household insurance. Check it covers skiing or hang-gliding etc if that's what you do.
We always use www.direct-travel.co.uk and find them very helpful. Twice we have had to claim from them under very different circumstances and both times they've paid out no problem!
Don't use the Post Office. Their policy is underwritten by Axa and I wouldn't deal with them again if I never travelled. I have an annual worldwide with NFU and they are brilliant but they only sell to household customers as an add on