@Julia522 Hi, I think the difference here is that through Airbnb we have temporary 'Guests'. With HMO these are classed as 'Tenants' on short or long term leases. For this a licence would be required.
Yep as @Moira21 says they won't be classed as tenants so no need to be concerned. I am not sure if it is a general rule but in my area you don't have to inform the council for B&B licensing/fire regs/change of use purposes as long as less than 60% of the rooms of the house are dedicated to guests.
i.e. My house has 11 rooms & I let 4 bedrooms. The kitchen is shared as is the bathroom and guest lounge so only 4 rooms are dedicated to guests only which is 36.3% so nowhere near having to be 'official'.
You should however declare any income etc to the taxman.