Having just had to search around for travel insurance myself I thought I would tell you what I've found out. Many insurers won't insure you past 28 weeks - the cheapest I found who do are "Leading Edge" 01892 836622. If I'd been less than 28 weeks when flying they would have covered me DH and DD for £22 (single trip to Europe) which is by far the cheapest I've seen.
I've taken insurance with Churchill who cover you up the 2 months before your DDate and the policy cost me £31.