MBNA were by far the cheapest when we went to thailand. 2 adults, 4 children, one in a wheelchair, and assorted medical conditions. They were over 100 pounds cheaper than the highest quote. If you have an mbna credit card and pay with that they give you another 10%off too.
Your "pre-existing" conditions will make the travel insurance cost more. Its not however just the quote cost you should be looking at; its the amount of cover as well that you would get for your cash.
It goes without saying that you must declare everything relating to all your families health re the hip operations and asthma because if you do not do so the insurers can refuse to pay out if a claim is made (and they have done this because they have said that their clients did not tell them the full story). If you are not certain I would speak to the insurers directly and get written confirmation accordingly.
You need also to read all the boring small print surrounding the policy.
Sorry to sound boring about all this but it is vitally important that you are adequately covered if you need to make a claim.