Choose a degree in a subject that you enjoy. Do not make a golden mistake of selecting something that you think will impress on an application form and then hate it/struggle with it for 3 or 4 years.
A degree demonstrates that you can successfully complete a longish period of study and you will also gain some valuable life experiences. If you are interested in flying/RAF, why not sign up with the University Air Squadron once you have enrolled in a course that you feel you will enjoy and have the potential to do well in.
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