You can rent out as many rooms as you like subject to the property not being overcrowded.
If it’s a council or housing association property you cannot sublet.
If it’s a flat subject to a lease, there may be a provision in the lease which stops subletting.
The tax question, you would need to ask a tax expert.
You don’t need consent from the neighbours.
If it’s a warehouse, you will need planning permission for change of use.
The definition of “room” would have its normal dictionary meaning.
It’s unlikely that emits a new company, any landlord will rent to a new company and a company cannot have Assured short hold Tenancy.
You pay tax on the rental income.
There is a mortgage you need lender’s consent to do this.
Any superior landlord is bound by the terms of the tenancy agreement and any superior lease as indeed you are