Foundation is a Basic Year for the Degree that you are going to pursue.
i.e. If you want to do a Degree in Engineering, before you enter into the degree course, you will have to do a basic foundation. Normally takes 1 year.

A-Levels is just like Form 6. It is the UK version of Form 6 where you will choose a few(4) subjects to study in i.e. Maths, Chemistry, Accounts, Law & etc. A-Levels is mostly meant for people who wants to go overseas to complete their degree in places like UK or Australia.

If you are really sure of what degree you want to pursue in now, do not waste time with Pre-Uni courses(SAM, CPU, A-Levels). Straight away enrol yourself into the Foundation year of the degree and move on.

If you are not sure, think again. It is not advised to go into Pre-Uni courses and giving the excuse that you will have more time to think. You WON'T!(I've been through this)