They used to publish means and standard deviations for core modules, but they haven't for the last few years, and I don't think they ever did for optional modules. To quote some SSLC minutes from last year when asked why they don't:
"The department feels that the numbers may be misleading to students due to scaling applied to the marks, students taking modules suited to their strengths, and a variation in difficulty of paper even with the same lecturer."
In theory it shouldn't make a difference with scaling and you should just pick what interests you, but if you're worried you can probably ask around in September once lecturers are confirmed to see what previous experiences have been like.