[ThePhysicist]

Hell no, even if this was legal under GDPR or the UK's own data protection law (which it definitely isn't) it would probably be illegal under basic law. Not an expert on UK law but in Germany there's the concept of "Unverletzlichkeit der Wohnung" (inviolability of your home), which I think is also a legal principle in the UK. Taking high-resolution photos of your living quarters is definitely a stark violation of your privacy and you should not tolerate that. If you're uncertain how to approach this I'd recommend to seek advice with a renter/tenants union, as they'll know how to handle this. Maybe it would be good to collect some futher evidence first, e.g. make a photo/video of the agent photographing your rooms and ask them about the app and how your data is being processed.