While what happens in the host's house is usually the host's responsibility (and I recommend everyone to take out insurance for third party liability, it seems that nowadays when someone gets injured they are automatically entitled to hefty sums), the freeholder/company managing the building is probably concerned about their liability in case of accidents involving Airbnb guests in the communal areas (lobby, lifts, stairs, walkways etc.).
This is understandable, as it appears most insurance policies for block of flats do not cover liability for short term stays.
Airbnb however have thought of that and provide the Host Protection Insurance:
https://www.airbnb.co.uk/help/article/937/what-is-host-protection-insurance
According to Airbnb, this also covers accidents in communal areas and should provide peace of mind to hosts, although I doubt this will be enough to reassure your freeholder.
Besides if you are letting the whole flat rather than just a room, there's also the issue of it not being used primarily as a "private residence" which is a common clause in leasehold agreements.
In the end, if the management company of your building has informed you short term stays aren't allowed, it would be a tough fight for you to win, it would probably end up in an expensive exchange of letters through lawyers, and it may not be worth your while.