Is it okay to mention joining the RAF on Ucas personal statement?

This is definitely okay to mention. Think of it this way: you are using the NHS (a national service program, funded by the taxpayer in order to make the country a better place for its citizens) to get a degree that you will use in the RAF (a national service program, funded by the taxpayer in order to make the country a better place for its citizens). Yes, the air force does very different work from the NHS, but both are national programs, and I don't think your Ucas application will be adversely affected. And after all, the RAF needs radiographers - where do you think they come from, if not university? Additionally, showing that you have a clear goal and that you need this degree in order to realize that goal can only help you get into the program of your choice. I would present it that way.