as far as i know, if you simply rent out a room but allow the person access to the lounge, kitchen etc ( what used to be called having a lodger ) then you do not have to pay tax on any income gained.
if howver you make the facilities for the exclusive use of the other person ( i.e. a self contained flat ) then you would be subject to tax on any earnings.
your local tax office or citezens advice bureau should be able to give you a leaflet about this.