If you are 'eligible' the Voda Business Unlimited Max is actually cheaper than 3 now. And you can even try it out with a 30 day Paygo (only double the cost). However, I don't think you will get a public IP if that's a requirement.You can get cashback deals for Unlimited Max from plenty of places which bring the prices down in line with what Three charge.
You have to pay the higher price and then claim it back later. A nice workaround though for better pricing.