If you already had teaching experience in the US (minimum two years), you'd have to find an LEA to sponsor you (not the easiest task) -- you'd have to go through "student teaching" again (observation, portfolio-building, etc.) and would likely receive a stipend while you were retraining.
Since you don't have any teaching experience, the easiest thing would be to go back to school to get a PCGE.
The ads for teaching assistants have required an NVQ3 or some sort of youth/playwork qualification, depending on the age you'll be working with. I think there's also a specific teaching assistant qualification you can get at most colleges. Of course, there's more leeway if you already have experience as an assistant in the States.
I'm not sure about being paid to train (other than in the above scheme). I would assume you'd need to qualify for tuition assistance or something as I've never heard of any other way people have had their training paid for.