dragonbutter - Usually they need a recognised Level 2+ gym instructor qualification (I think) and they do some training themselves. If you're already an instructor/trainer and have an accredited pre/post natal qualification you can skip their training. Speak to Emma Redding though, she's great and I think given your physio qualification and experience she would be very willing to consider it.
It's a licencing operation, you pay an annual fee - I made mine back very quickly - then it's really up to you. My daughter comes with me most of the time (she's 21 months, bribed with raisins and breadsticks to sit in pushchair) so I can make some extra cash without paying the nursery fees.
At the moment I run 4 classes per week (summer is busy) and I put in quite a lot on top of that in local marketing, PR and enquiries etc. I've spent a lot of time really getting to know the local health visitors and midwives and go and do talks at all the mother & baby clinics, which is a really good way of getting people along.
It works well for the rest of my business though, I'm a PT specialising in pre and post natal, so I often get clients from Buggyfit - I also write for a couple of magazines on post natal fitness so it all helps.
I've had more around 250 mums and babies come to classes in the first year, met loads of great people (occasional grumpy snooty mare) and really enjoyed it. Still doing at now at 6 months pregnant and not planning on stopping any time soon!
If you want, contact me directly via the Buggyfit website, I cover Windsor & Ascot. If you're not miles away you're very welcome to come to a class (for free obv) to see what it's like.
Not working on commission, not related to Emma - just love my job!