I take it this is for longhaul international flights, as guide dogs regularly travel in the cabin on UK domestic sectors.
Usually the seat/seats next to the person with the guide dog are blocked to allow them some extra room. Assuming that is that the flight is not full. The guide dog also lies at the feet of the blind/partially sighted passenger.
As for informing the Captain, this should be done, both as a matter of courtesy and also via the loadsheet - showing as a Pet in the Cabin.