Have you tried Easynet at all ?
They have a long history in education and local government. I've not had dealings with them for the last 18 months or so, but you couild always ask them about billing options for the public sector.
From memory I already know that they used to offer payment options to public sector clients. Whether they still do I'm not certain, and I doubt that you'll find a counterpart product of theirs for the same cost as 1&1 MS Business Pro, but it may be an option if all else fails.
Having said that, have you tried the LEA at all ?
Most schools have their ISP arranged by or through the LEA, but all schools also have their own address along the lines of schoolname, county reference and .sch.uk suffix, along these lines:
http: // www . school-name . northumberland . sch . uk/ or http: // www . school-name . worcs . sch.uk
If you ask around and do a bit of nosing you could find yourself coming up trumps.
Your local authority ICT advisory team (aka tech support) should be able to help with this.