Owning your own home

I can't help with your query but every time this floorplan gets posted I have a little chuckle at "not heated corridor" and wonder why the agent felt the need to mention that :-D

Someone told me that flats need a separate door between kitchen and front door. 5 flats in this mid 19th century building

Pretty sure you'll need something as that forms an escape route. It may be a fire door or a sprinkler system or just a load of fire alarms (probably not that one).

I spent a long time looking up various impenetrable regs when I was looking at my place, speaking to someone at building control was an easy way to find out what was and wasn't allowed (and what compromises they would sign off on).

Not sure how high your flat is. 3.18 b ii here might be relevant
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/937931/ADB_Vol1_Dwellings_2019_edition_inc_2020_amendments.pdf

Your layout is fairly unique. Usually the only way an escape route could lead near/through a kitchen is if you have a mist system, or sprinklers installed. Open plan room = corridor/escape route.

You would probably be expected to defend the front door from an overnight kitchen fire so the bedroom occupants had time to escape.

We've used Automist previously which was £2960 + VAT for the following:
2 x Automist smartscan systems
2x wall mounted emitters
2x hard wired Aico
Heat alarm
24 months warranty

Shout if you want other Automist details: pump location and water pressure might be tricky, etc.