As far as I'm aware, M100 is three years and M101 is four years. I think that M101 just lets you take more options. It's basically just another year of a Law degree.
Haha this puzzled me when I applied last year as well. Durham had M101 as their course code, but the LLB only lasts 3 years. All my other choices were M100. I don't know why it is .
haha when i applied two years ago i applied for bothh m100 and m101 and got offers for both (both from ucl)- i was baffed because i thought getting an offer for one would mean i couldn't get one for the other- i picked m101 (to no avail in the end) but now i think it's a bit of a wasted year (career wise)- unless you jsut wanted to read more modules out of law-love; or get into academia.
it really depends on the uni, some M101 course are for Senior Status llb (some already with a BA wanting to do an LLB), some are llbs with extra like year abroad, sandwich year extra. which uni were you talkign about?