Hi TheCollection!
I booked mine just before I turned 16 weeks. I searched on the site rubles has linked to and contacted a couple (only 2 were close enough for me to feel comfortable with). One of them I already knew from a childbirth group I go to and we went with her (the other didn't reply until after we'd booked cos she was away).
I too would book asap not only to ensure you can find one available but also to get the maximum benefit from it.
I have absolutely no regrets whatsoever. It is the best thing I've done. I know that no matter what happens later in my pregnancy or during the birth (e.g. if I end up going into hospital for any reason) that I won't regret spending the money.
Personally I would have a good chat with each on the phone and then have a consultation with the one you like best (usually a charge for this). Ask that person all the questions you have, what's included in the price, etc and make sure you're happy with that choice. If you're unsure have a consultation with the next favourite iyswim. I think it is very important you feel comfortable with your choice.
Tbh the lack of stress, the one hour antenatal visits, the discussion and balanced information (as opposed to 'trust policy') and the continuity of care are worth so much. I also know that the postnatal care is far, far superior to that offered by the NHS - visits every day in the first week, every other day in the second week, and so on.
Also with the ones I've booked it's an all-inclusive price so if I need extra visits for any reason they'll come out, it doesn't cost me any more and I feel very very supported (I've already had 3 extra visits related to me feeling down & anxious and 1 meeting at the hospital for a birth debrief).