You can use 2 years separation even though you have lived under the same roof. During the 2 years you must not have lived together as husband and wife, you must not have shared domestic tasks, eaten together, cooked for one another or shared a bed.
OP, as Tets said, you will need to ensure there is no indication of marital life - do not even do his washing, or cook.... you do not say if there are children of the marriage, but if you and STBX are in agreement, I would suggest that 2 years sep is best basis, as opposed to UB which can be unpleasant for Resp to read and may cause animosity, esp if he is still living in same MH. Is there a formal Sep Agreement? Probably not. This can hold sway with the courts, though if your STBX is in full agreement, and you both just want to get a divorce, use 2 year sep grounds. But as Tets said, this does need to be based on complete seperation of marital life whilst living under same roof. You will need to post again re the financial side using S25 criteria.
Suggest you read up on here re previous posts. Hope this helps