This is why i asked the question on owning versus renting the other day. Look at your contract and see if you have the right to sublet the spare room, most landlords do not include this in the standard contract as they don't want the tennant to sublet the room and as such house to a heroin addict or whatnot so you could actually be doing yourself a bit of harm by telling her a mate is renting it.
The landlord has the right to inspect the entirety of the property but has no legal right to inspect anything that was not included with the flat. I think that if you have a built in wardrobe they have a right to look inside (that's me fucked on two accounts) but if it's a standalone job then they have no legal ability to look inside regardless of their suspicions. Is your grow a tent or in the imddle of the room? If a tent just chuck every item of clothing and cardboard box and well, every posession you own around the thing, so long as there is no apparent damage then she has no legal right to tell you what state of cleanliness a room must be in, tell her it's where you store everything to keep ya bedroom tidy.
certainly wouldn't fancy being in your shoes though. From experience with my landlords, my inspections are generally just wear and tear of carpets and furniture, and not much more, but who knows what yours is like.
I would also strongly advise against putting a lock on an inside door, unless it can be installed without screws or any form of modification to the structure. And again with the point about the ability to sublet.