I'm not convinced a degree is worthwhile for many people, but especially not ATCOs. You would be far better served getting some work experience in a numerate role while waiting your ATCO training slot. Having said that ATC is very competitive and the odds are stacked against anyone, so have a backup plan and maybe educate yourself relevant to plan b. I have seen PhDs not making it and a GCSE only being among the most natural controllers. Most of my middle managers seem to be uneducated, but driven. Many of my academic colleagues don't feel the need to prove anything and are happy "just" controlling.
If you have a mature enough outlook to get straight on the (very basic) ATC training course, you would be much better off just going for it and having an extra few years' pension contributions and no student debt. Before anyone cries "sour grapes" I speak as an Engineering graduate from a time when degrees were still a rarity.