I recently travelled to Maldives when I was 28 weeks pregnant, in the end we used the travel insurance offered by my partners Lloyds bank account - he just got me added for the trip. Key things to ensure it includes is cover not only for early birth but also that the baby is covered for medical treatment too if you do have him/her abroad, plus repatriation costs if they need to fly you home. I have also heard about Ravenhall. Looks like you pay anywhere from £25 -£40 for a decent policy.
Also, I got a fit to fly cert from the Docs just in case and BA DID ask for it at the check-in desk and as I got on the plane so definitely get one of those, even though the advice I read was pretty non-commital on the need for it.
Enjoy your hollibobs!