Sounds like you'd have to be told that it's medically unsafe for you to travel before you cancel it.
Have just booked a holiday to normandy and will be coming back when I am 36 + 6. France is closer to us than london is, so it's not too far away. Managed to find insurance through a company called raven hill, they were really good, so if you can't cancel it may be worth insuring against pregnancy related stuff through them.