Unfortunately it is notifyable work for both the stack connection & the window/extract fan; this is a direct extract from an LABC website:
Q: If I want to install fittings such as a WC, shower, washbasin, or kitchen sink within my home, or any other type of building, will the Building Regulations apply?
A: YES - if the installation of the fitting or fittings will involve alterations to, or new connections to, a drainage stack or an underground drain. You should also check with us to find out how much information is required about the fittings you intend to use.
I formed a new utility room which involved connecting the waste into an exsting, nearby stack, I did’t pre-notify which, strictly speaking, I should have but as I had other inspections pending on additional building works, I just mentioned it to the LABC inspector the next time he visited; he glanced in the room, looked at the fan (no window in the room!) & stack connection & said “yea that’s OK” & that was the end of it!
It’s not a big deal, give them a ring & explaing what your doing; they may not even bother to inspect but if they do, there will be a charge.
Just thought; I think you must also have a hand washbasin in the room but without checking, I'm not 100% sure !