The thing is how on earth you find people who are available, and not already on the lists of various agencies - PLUS - how you get in with the schools.
Can I ask if you are already a teacher or teaching assistant? The schools have people who get told somebody is ill around say, 7am - they then have a very instant need for staff. I'd get a phone call at maybe 7.45 - and I would be 30 minutes drive away from a school, so the chances of me getting there on time are small. Clearly, the agency try the closest first, By the time they get to me, all the other people have said no. Many schools now have their own cover staff local lists, or even permanent salaried staff who do cover. A new agency cannot provide any better service than the schools already have, as they will be using the same staff to fill cover? Unless you have access to extra teachers and teaching assistants they others don't, that mean you say yes more?
The schools don't care about the quality of the staff at all - they breath a sigh of relief when ANYONE walks through the door.
In addition, it really is a horrible job. You get the worst work to do. Teachers prepare schemes of work and lesson plans for cover teachers coming in. They are always dull, boring and unexciting for the kids - the better lesson plans being kept for the teachers own use - so they are fillers, full of the stuff the teachers themselves hate. I'd estimate that at least 80% of the stuff I did, was pointless, and many of the schools made no secret of the fact that they paid no attention to supply staff, once they were there!
If somebody started an agency who promised me regular local work, I'd have jumped ship immediately - the two agencies I got work from were useless, and I was just the next on a list. Can you offer people regular work, at a rate that they will like? 60 miles a day with no payment for travel stinks.
Some schools are so bad outsiders would not believe. Not allowed computer log in, not allowed to go to the toilet, not allowed to buy food, totally ignored, just sent to a room full of kids who see the supply teacher as a challenge they always win. Luckily, I was fine with all this - but some teachers get too stressed - and as an agency you get the call saying they never want that person back.
One day the school asked if I would be back tomorrow - I told them that I would never, ever, be back because this was the worst school I had ever been to!
I cannot imagine why you want to go into this, unless you know a pile of people you can put on the books.
