Each course (aerodrome, approach, radar) will be about 3 months and leave you £15-20k out of pocket. There would be no guaruntee of passing, no refund if you don't, and no guaruntee of a job at the end. If you are lucky enough to find a job (more likely now that more people are leaving for other careers) then the pay is woefully low, especially relative to the initial investment.
Some airfields sponsor a small number of assistants to do the courses, but don't take a job purely for an off chance of sponsorship, take it because you want to be an assistant there.
Whatever you decide, good luck!