I just called one yesterday. I don't know what is available in the UK but I thought I would tell you something about the Canadian ones just in case you run into it.
I was told that you can get the insurance and you don't need any notes from your doctor. They will only cover you if you are not a high risk pregnancy and only during a certain time in the pregnancy like spicy orange said. If you happen to need the insurance, then you need to show them the note from the doctor saying that you were not high risk. Then the insurance company will determine if they will approve the claim.
That seems sketchy to me. I have had MCs before, so I bet if I needed the insurance they would say that I was high risk even if I was carrying a healthy baby now.
When you find a company just make sure you look to see if they are the same.