may need to resit year 13 exams but accepted unconditional uni place?

So I have accepted an unconditional offer to study at uni form September (an offer which I received based on my GCSE results and generous predicted grades from teachers who had hope I would improve this year, based on my GCSE performance, even though I have struggled with A levels) but the hope teachers were holding out that it would all click and I'd do well again has not come to pass and now it looks like I'm on set to not do well at all in my A levels but I do have my place for September already confirmed... obviously because it's unconditional my grades won't determine whether or not I am accepted but I am worried that my postgraduate options will be limited if I flop in my A levels, even if I do do well in three years time when I finish my degree - does anyone know if it would be possible/would it be sensible to go do my first year at uni and then resit my exams next year, summer 2018?