Tough? Depends on the individual going through the process. Some will find it difficult, others won’t. Some who find it difficult will get through to the job, some who found it easy will not get through- and vice versa.
If you’re in year 11 I recommend working hard at your maths and English subjects and exposing yourself to as many teamwork, decision making, leadership type situations as you can. Sports teams, debating teams, cadets, scouts, guides, customer service or other team type jobs, etc are all good things to stay involved with to develop the non technical skills required. Most people can be taught the ‘stick and rudder’ elements of the job. It’s the non technical skills that get you in the door for a cadetship.
Good luck