You can but if you're over 19 you will normally have to pay fees - depending on where and how you take this, these may include tuition fees and/or exam entry/registration fees. If you don't have a level 3 qualification already you might be eligible for an Advanced Learner Loan (ALL) to cover some or all of these costs. If you do have a level 3 qualification already you may be better off looking at Access to HE courses or degrees with a foundation year, as you can get an ALL regardless of whether you have a level 3 qualification already for an Access course (and if you complete a degree afterwards, the ALL gets written off with an Access course!), while for a foundation year you get funded by SFE in the normal way for degree level study.
Note that if you're planning to do science A-levels they get very expensive to do as an independent student as universities normally expect you to take the science endorsement (practicals) for all science A-levels taken, and this can be easily £1000 or more per science A-level taken, on top of any other tuition fees, exam entry fees etc. Which is another thing that doesn't factor in with foundation years or Access to HE courses (although I am aware for some unis access courses aren't routinely accepted by themselves for science degrees due to not exactly covering all the necessary material).