Psychology compliments Biology, Chemistry and Physics and it is perhaps more closely related to medicine than English. However, Psychology is deemed to be of limited suitability whereas English is looked upon more favourably, it's also good to have a balance of arts and essay subjects. I did English Language at AS level and thoroughly enjoyed it, as did most of my friends who've studied literature as well. If I were you I'd take the English simply because you're applying for a very competitive course and so having four traditional subjects at AS/2 level might put you in better stead. Then again, I haven't studied Psychology; it does have the biggest drop-out rate at my sixth form though, maybe an indication of something...