3 bedroom house with converted loft but no license from the council rent as HMO

Hi,

I'm in the process of purchasing a house for buy-to-let. The house is advertised as a 3 bedroom with loft, with 4 students living in it.

The house itself is in a good condition, and the loft conversion seems decent, insulation and a window are provided and a carpet on the floor is fitted. However the LL hasn't acquired a HMO license as yet, and I will need to inquiry for one should I buy it. Furthermore, I've been told that the loft conversion hasn't been done through official channels i.e. the conversion is not registered with the Brighton city council (where the property is).
Now, as I mentioned there is man living there and it seems to be in a quite good condition. Obviously it cannot be regarded as a 4 bedroom house.

I'm thinking of renting it through a lettings agent.
What are the chances of the council prohibiting me from renting it as a 4 bedroom house?
Could I rent it as a 3 bedroom house, with a price of a 4 bedroom and inform the people that "one more person can stay in the loft" ? Can this cause any troubles down the road?
The estate agent told me that he doesn't think that the council might be fussy about regarding the loft as an extra bedroom.

Thanks