Lop off 20% for working term time only. (So you get paid for the equivalent of working 4 days a week)
If you do less than absolutely full time, Mon-Fri, say an hour and a half less a day, take off another 20% for that. You will probably still end up working every lunch and break. And before school. And after school. It's all about the Children, you know. And somebody has to be in the playground whilst the teaching staff have their breaks or meetings.
Then take that total and divide it over 12 months and you'll have an idea of the gross wage.
Take off your pension (worse than the teachers' one), tax, NI, etc. Allow for the costs of travel and being suitably attired in expensive professional business attire. And whether it has implications for your length of service, as support staff work isn't counted, so your length of service is zero.
Think of how much you get as supply now. Compare the two. And take into account you don't have any rights that teaching staff do, because you're going to be on a support staff contract. And who goes first when a school is skint or needs to pay the consultancy fee for a logo and uniform rebrand as part of the dragging it back from a crappy Ofsted report and covering the cost of the new and improved shiny young things that are going to be part of SLT? That's right, support staff.
Then there are the less tangible things. You're not a teacher, you're support. Some people will speak to you differently - and not just kids and parents, I mean SLT in particular.
In addition, if it's like the standard Teach First, you'll also have to attend study groups on Saturdays and Sundays. And if you leave, the contract says you owe them money. A fuckton of it. And they're targeting frankly, the shittiest schools. No nice little outstanding secondary for you - massive amounts of deprivation, violence, poverty of opportunity, aspiration, a culture of desperation, exercising crowd control - and hopefully persuading a few disenchanted and disenfranchised kids to do five minutes work. Assuming you get a head who thinks you're there to be a cheap cover supervisor. After all, they'll get a teacher effectively - no problem you doing all the planning and marking as well, is there?
By this point, you will have probably worked out that you'd be better off getting a job in a supermarket, as you'll have more opportunities for overtime, plus the ability to take holiday during the year, not just when everything is at its most expensive. Or stay as agency. You'll be treated better - and when it comes down to it, you can always walk out of a booking, which you definitely cannot do with TeachFirst.