The courses you get depend on your job in the army. If you want training you can use later, sign up for a trade (army mechanic, electrician, welder...).
If you end up in an unskilled job like the infantry, you won't get much education at all. You don't have a right to move from the infantry into a skilled trade once you've joined up.
Joining aged 16/17
If you join at 16 or 17 and don't have good GCSEs then you get to do some basic Engish and maths courses, but they're not as good as GCSEs and employers don't think much of them. Education at a local college is better.
You also do an apprenticeship, but if you're in a basic frontline job like the infantry then the apprenticeship is just your basic soldier training, so it doesn't help you much.
If you sign up for a trade instead, then the apprenticeship can be quite good, but the army tends to give 16 and 17 year-olds basic jobs in the infantry.
Why wait to 18?
If you stay in school at 16 and 17 to get good GCSEs (and maybe go on to do BTECs or A Levels or Scottish Highers) then you can choose a technical job in the army. Then the army will give you better education and training.
In 2016, soldiers were asked what they thought of the opportunities they get in the army. This is what they said:
Find out how long you can stay in the army if you enjoy it...