A lot depends on the kind of job you sign up for. Some army jobs can lead to courses in a trade, others don’t offer very much.
If you want training that you can use later, sign up for an army trade (mechanic, electrician, welder…).
If you end up in an unskilled job like the infantry, you won’t get much education at all.
Joining aged 16/17
If you join at 16 or 17 and don’t have good GCSEs then you get to do some basic English 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 it can be better to wait till you’re 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.