You actually don't need to pass all the compulsory modules, some of your compulsory modules will be what's known as 'Pass for Progression' (PFP) these modules you do need to pass, your school should let you know which modules, if any are PFP. Any other module can be failed any you'll still be allowed to continue on an honours degree providing you still pass a total of 100 credits, obviously this makes any module over 20 credits a PFP module, even if they aren't technically one.
You can also recover credits lost in later years, so say you only pass 90 credits this year, if you pass at least 110 next year you can take 10 credits from next year and count them as this years credits and hence stay on the honours degree.