insurers will not cover theft where there is no sign of forced entry and the house is in multiple occupation. The other tenants, and indeed the landlord, have free access to take whatever they want at any time.
It could be useful to know how old the house is, and if the doors have ever had a lock fitted before. Old houses often had low quality sashlocks fitted on internal doors, and these can be upgraded to a more modern lock without spoiling the appearance of the door, though the mortice will need chiselling out as the new lock is usually thicker, so it needs to be done by a skilled person to avoid the damage that the landlord is presumably worried about (unless his reason is that he likes to snoop about peoples rooms)
The knobsets you are thinking about usually need a large hole to be drilled in the door to take the mechansim, and are not much good anyway
It is very common for students in shared accomodation to be robbed and burgled, they are not used to keeping things secure and any one of them, or their friends, acquiantances, drinking buddies etc might leave doors and windows open, or be the culprit. You might consider a secure cabinet for the laptop but it would need to be screwed dow, probably to the floor inside a wardrobe, since I expect the landlord would not want holes drilled in the walls. But anyone with easy access could snoop round and come back later with appropriate tools.
BTW it might be worth considering how much the landlord will charge if a lock is fitted and he claims it has damaged the door, and how long it will be before he finds out. A yale lock leaves quite a large unsightly hole, but a mortice lock, skilfully done, can be quite unobtrusive
