@Mark10 is this an upper floor leasehold flat where the freeholder owns the ground floor entrance lobby? If so, then its the freeholders responsibility to do a fire risk assessment of the lobby and act accordingly, (smoke alarms, fire doors etc). If the building is a conversion that doesnt meet the 1992 building regs and more than a third of the flats are let then the building will be a s257 HMO and that requirement is in that legislation.
If you cant persuade the freeholder to do that, I would suggest that as a minimum you put battery powered interlinked smoke alarms in the lobby and your flat as you would have no authority to do any hard installation in the lobby. If the freeholder wont act then you may also have to assume other fire safety responsibilities as per the Fire Safety (England) Regulations 2022.
Upstairs purpose built self contained leasehold maisonette. Exclusive access to entrance and stairs which lead to it. (Leaseholders responsibility to maintain).
I like the idea of battery connected smoke alarms.