Absolutely also going to recommend studying an Art Foundation course, as @totallyfine says it's free if you're coming straight out of A levels at 18. Will do nothing but benefit you. Don't worry about being "behind" - you'll find lots of people on art courses will have come off an art foundation year.
That being said, it's absolutely possible to get into an art course without formal art education, but it doesn't half help. The main thing that formal arts education will help you with is having development, design and iteration work in your portfolio, which is one of the key things universities will look for. They are likely to also look for examples of drawings from life (e.g. still life, figure drawing, technical drawing), imaginative work (combining ideas in a an imaginative way) and possibly may look for art in different mediums (including paint/pencil etc. but also things like model and sculpture).
Personally I would try to make sure to have examples from all of those categories (and combinations of the 4) in your portfolio before applying.