Took gap year between as and a2 help!! :'(

I did my EDEXCEL (IALs) AS level exams in May/June of 2016, took a gap year, and now I want to finish my A levels by re-enrolling into school and writing my A2 exams in May/June 2018. Am I allowed to do this? Will my AS level marks be carried forward to my A2 or is it impossible for me to complete my A levels now because of the gap year I have taken?

note that this gap year wasnt my choice but i was forced into it and now im forced to go back to complete my a levels i will take chem math and physics and ive forgotten every single thing ive learnt in AS level how hard is it going to be in a2 with only 6 months in hand