The problem here is that you are “using the flat mostly for residential purposes”.
If there is condition in the lease (not your tenancy agreement but the lease with your landlord has) that this should not be used as anything other than for residential purposes and/or that you’re not allowed to run a business from it, then your landlord is in breach of his lease by allowing you to do it.
It was irresponsible of the agent to tell you that you could do this if that term is in the lease.
I can tell you that it’s more common that these restrictions are in a lease, than not.
In respect of it being illegal, it’s only illegal insofar as it’s a breach of the terms of the lease which could end up with the owner of the property in court unless he stops happening what is happening.
Notwithstanding any of the above and whether this is allowed or not, neither the other tenants or the freeholder are allowed to harass you or your visitors because that in itself is a breach of their own lease not to cause nuisance.
The fact that you may only be talking to people and not having more wear and tear is not relevant.
Google is a very dangerous thing.
The article that you sent is incomplete because it makes no mention of the terms of the lease of your landlord and that is central.
I’m sorry but based upon the lease, you simply cannot do what you are doing. Sorry.
Can I clarify anything else for you?
I am happy to answer any specific points arising from this.
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If you still need any point clarifying, I will still reply because the thread does not close.