B1
It's very much up to you how you sit the exams. As long you pass them all within 4 attempts and 6 trips to the exam hall, oh yes, and all in 18 months from the date of the first sitting of course.....
If you fail any you normally miss the deadline to resit the following month due to the length of time the CAA take to mark and send out results. So, if you sit exams in Jan and fail one, the earliest you will be able to resit the exam will be March.
I don't see that there is any problem with you starting your second module when you have a couple of exams outstanding from module 1. It's up to you whether you leave any resits until after sitting your module 2 exams or sit them whilst studying module 2. As I said, it's up to you. Your FTO, in your case Bristol, may advise you on which way to handle resits but do whatever you're happy with.
I did mine in several attempts two years ago. That's how the system worked then, I haven't heard that its changed.
I hope it makes some sense! Good luck with the course.
Regards
RM