My advice would be to do it in 2019, you can study for the UCAT over the course of 9 whole months, but make sure to prioritise your school work though. Even if you don't well in the UCAT in 2019, you already seem to be willing to take a gap year so you can do it in 2020. If you personally think that your school work will be too hard on you (I dont know what subjects you are doing or whether you find any difficulty in them) then I would suggest taking the gap year