Why do you want to leave teaching? As a teacher you can earn a lot more than a childminder, especially once you make your way up to deputy head. As a teacher you get long holiday periods, so you are already spending a lot longer time with your DS than other working parents. As a teacher you get maternity leave and maternity pay, and you may get opportunity to go back to work at reduced hours/days. While you are a teacher you will be keeping your knowledge updated. Once you leave teaching... you may never get back in again. CM's are self-employed. Some years they may make quite a bit of money, while other years they may make very little. It's quite an insecure job, especially at the moment. Think long term - do you really want to leave teaching?
Set-up Costs: Depends on what you already have in place at your home. I would expect you could start up for under £500, excluding advertising costs.
Ofsted Registration: Supposed to take around 12 weeks. I'd suggest allowing 4 months.
How long for first mindee: alas, that is a how long is a piece of string question. You may be lucky and fill your spaces quickly, but you may not.
£3.50 per hour sounds low to me, but you are right in saying that it varies by area. Where I am childminders charge £4.50-£5 per hour average for a full-time under 5 child.
I would suggest reading as many threads on this board as possible, so that you get a better idea of what you will be getting yourself into. There are many threads on here with regard to things such as how much to charge for various things, about EYFS, Ofsted in general, suggestions for places to advertise, details about insurance providers, what paperwork needs to be done, how to terminate an agreement with a parent, how to try to get money owed by a parent, all sorts of things.
I would say that given the current economic climate, if you are working now to pay your bills, don't leave your job. Continue being a teacher - it is a more secure job than childminding.