There is no direct entry officer route into 21/23, except for those already serving as officers with 22. Ex-regular officers from the rest of the Army need to pass the regimental commissioning board, but won't get a date for the board with less than three years service in 21/23, unless they are truly exceptional.
To get to the regt commissioning board with no prior military service will take you about five to eight years, perhaps more. Life as a Trooper -> Corporal is great, though, and really all the officers do is a lot more admin, and not so much of the fun stuff.
Remember, that as a Corporal you'll be a patrol commander, off with your 3-5 lads for up to two weeks with only the radio as contact with the boss(es). That's as much fun as it gets in the Army as a whole. When you've got that hacked, then consider going to the officer's mess.
You'll find that your average trooper in 21/23 has a degree, speaks at least one foreign language fluently, has at least one major skill such as diving, engineering etc., and can run a marathon in under 3 hours 20 without having to train, and I'm being quite serious.
To be an officer takes a lot, as a result.