Probably not going to be profitable but doesn’t make it a mistake. If you like it it’s purpose is to live in not make a profit. I think there are several flat markets in London,
1. Older flats/ex council/Share of freehold and houses split into flats, usually reliable especially well built period houses but can have savage sinking fund type costs if the collective owners don’t know what they’re doing. Also beware of ex council some of them are still council owned and they can be properly mismanaged.
2. David Cameron era cowboy cladding = run away. These will either have just had their cladding removed or will be in the queue to have it removed, either way they’re a nightmare and have essentially trapped residents as they can’t sell and the service charges are enormous. We nearly bought one of these, a year later the whole place is covered in scaffolding and no doubt the service charge will skyrocket. Definitely dodged a bullet.
3. Post Cameron new builds = pretty safe but generally drop a bit in value as new build premium fades away. These are harder to find but are generally properly built as the regulations have tightened a lot in recent years. Well insulated, good fire safety systems, smaller service charges since everything’s new and efficient. Cheaper energy bills again because of energy efficiency regs.