You should definitely go to uni first off, even if it's to do something like macramy or basket weaving - geography is a particularly well kept secret, not too many hours & damn near impossible to fail
If nothing else, uni will give you a bit of time to see the world with the advantage of a safety net under you and get a bit of life experience, which is something that frequently, but not always, shows through when you look at DE / Grad entrants side by side. Most importantly, as the RAF is currently being flogged to death, it will give you 3 of the best years of your life and that's no exaggeration.
But on the practical side, if the RAF decide to grill you on why you went to uni rather than joining straight up, the answer is easy. It's called plan B, something you will find you need frequently out on leadership exercises at IOT! There are no guarantees that you will get into the RAF in any of your chosen branches or that you will pass training. Also, there are no guarantees of a full career. You may find yourself in the brown and smelly if all your plans come unstuck and you find yourself late-20s having been chopped off the Typhoon OCU with no other offer of RAF Police or the dole queue. A degree will give you a few skills to bank away, just in case!
Do uni, drink lots, play sports, fly - if you can and chase girls. And if you have any time, a bit of work here and there tends to be appreciated by your lecturers! Trust me, you know it makes sense