I didn't get the grades I needed at AS to do medicine but I hope to improve it a bit in my final year, A2 and see if I can apply next year, but I'm applying for Diagnostic Radiography for now as a back up.
If I do get the grades I need then I will apply for medical school, and reject my radiography offers, and take a gap year. Can I still guarantee my radiography places from before?
This would not work in UCAS. If you want to apply again the following year you can’t be holding any offers from the previous year. If you get a place on radiography and good grades, you could try talking to the university and explain what you want to do and ask if they would make an offer again the following year. They might say yes. Otherwise it would be taking a chance that a fifth choice would go well (though these professions tend not to like accepting people who really want to be doctors).