If, as it sounds, you're breezing through A-Level chemistry (ie getting the concepts as they appear in the classroom and not having to commit to any large amount of reading round the subject) you'll breeze through the first two terms. Put in the work and you could feasibly end Prelims with a Distinction.
2nd year is where it begins to ramp up and you can quickly get left behind, but some concerted effort at revision can sort that out.
As for lectures and understanding there and then, its a rare thing for many, particularly in physical where the pace is particularly quick. You'll find plenty of people who lay down the 'i'm so screwed, i don't understand a thing' chat so if you feel that way you definitely won't be alone.
Even the class of what I'd call 'super-chemists', (top 15 or so in the year) have their off days so you really have nothing to fret about.
Put in your application and come and have a great time, so far it's been the best two years of my life