A Level Further mathematics

Hi all,
I am currently in year 12, i opted to take single maths rather than further. I am now regretting that decision as i have really found myself at a level and would like to pursue a numerate degree at uni, unfortunately its not an option to pick up further maths at this stage.
I am currently doing 4 a levels and an EPQ and am therefore hesitant to try and self study.
Taking a gap year after year 13 is definitely something i want to do and I was thinking that it could be feasible to self study FM A2 and STEP in said gap year although i don't know where this would leave me in terms of uni applications. Would i be able to defer to 2019 even though i would have 2 sets of conditions to meet (4 original a levels 2018 and FM 2019) or would i be better off applying after I've left school (this could be problematic re admissions tests/interviews as i plan to travel for some part of my gap year) or just try and bite the bullet and do my best to self study?
sorry for the poorly written and very long winded post, hopefully someone can help