Firawla.
If you have a complete and signed inventory for your tenancy then all you have to do is to return the property in the condition in which it was stated when you took it over. ideally with pictures. If it was clean for you, get it at least as clean for when you hand it over.
Go and talk to the CAB and see what is what. Take a copy of any inventory you have, and tell them your deposit is not as yet protected. AFAIK, LL can't evict you if there is no deposit protected, but if you have handed in your notice, that is different.
If your deposit is substantial, then consider getting an independent inventory clerk do you a check out inspection/report so that you know areas that need to be addressed by you before your official check out.
Oh and who was it that said that the security deposit belonged to the LL? What utter bollocks! Clearly a crappy landlord! It's the tenant's money, and never stops being so. it's a security deposit to be claimed against in the event of any accident or damage.
In order for the LL to claim any of it, through the dispute system, they have to PROVE that they have a right to charge x, y or z. If they can't, they don't get to keep it.
I had a LL that tried to do me for over half my deposit, nearly £500 in all. I did a statutory declaration ,sent all the documents I had off to the TDS and they ruled totally in my favour and were scathing about her behaviour.