Seriously, I work in this field and the only sensible advice here would be, do not move into a property where the landlord insists on quarterly visits. It is not that common. And most landlords will not be cool if they discover a grow in their UK property. They will report it to the police in almost all cases. Some letting agents do insist on a mid-term (or mid-tenancy) inspection, but it's only one inspection and you could keep a door locked and tell the clerk that you're roommate is travelling (most times they'll just stick a note on the report that the room was locked and that will be the end of it, assuming their boss isn't too pedantic - as someone posted above, the inventory clerk or agent is not really interested in what you do there). But even then, I wouldn't risk it if I were you, as they might insist on returning when the room is accessible.
Many agents do not insist on mid-term inspections and that HAS to be the way to go if you're planning on growing. If you are dead set on this place and there is a loft/attic, there's a great chance they won't look there, but if it appears easily accessible and more importantly, if it features in the original inventory inspection report for the property, they definitely will do. I know this because I would! If it is not featured in the original report, chances are probably slim to none that they will even open the hatch. If it is, you're shit out of luck. The job of the clerk doing this inspection, is to compare the current state of the property to the report produced when you checked in. So as I say, the biggest issue will be, is the attic in that report. To be fair, it's unusual to include an attic, unless it's been subject to a conversion and is listed as a bedroom or something.