BGS will be relaxed about this - it'll be up to you to book the revision course when you feel the time is right although best to give a few months notice to be sure of a place on the course.
Think I'm right in saying that the CAA 18mth rule kicks in after you take your first ATPL exams i.e. after your Mod 1's with Bristol, you then have 18mths in which to pass the Mod 2 subjects. No rules on how long to take on the Mod 1's however keep up a good pace otherwise taking too long may result in the earlier study becoming a little hazy. Hope this helps.