You cannot withhold rent to make any sort of a point.
That just gives her a clear legal reason to evict you, so thats stupid.
You should pay your rent on time, again thats giving her reason to evict you - sent it up via a DD or SO.
The way to alter things, if it is possible to do this and it might not be, she could simply not renew your contract once it is up, and evict you that way...
Is to find out if shes breaching her HMO licence.
She will be licenced for a certain number of tenants/rooms - it sounds like at least originally, that was 3 rooms, and she has now turned the communal living space into a 4th room. If she is still licenced only for 3 rooms, then she could be in the shit legally and with the local authority that issued the licence.
They don't HAVE to provide a communal living room...
But only if the kitchen is big enough! You'll need to contact your local authority to find out a/ if the kitchen is big enough to allow the landlord to not provide a communal living room and b/ if there is a licence for 4 rooms.
Then you may have something in your favour - however your history of not paying rent on time and now withholding rent is not going to help matters at all - I would suggest you pay rent on time, every time and pay the outstanding money now, or find somewhere else to rent.
If you do try to take on the landlord there is a good chance that even if its clear they are not following their licence, they will find a way to evict you.