I was a Custody Sergeant in a large County force and then transferred to London Metropolitan. The cell complex is pretty much the same but varies in size. In the counties we had 7 cells and a detention room for juvenilles. In the City we had about 12 cells and 2 detention rooms. The prisoner is brought in from the outside to the outer office where the Sgt hears the evidence and authorizes custody, the prisoner is then searched and his property bagged. He is lead from the custody office through a heavy barred gate much as you see on the cells in US Westerns. Behind that gate is a corridor with cells off of it either on one side only or both sides.
All the cells are the same, heavy metal dark brown door with a hatch in it that can only be opened from outside and allows meals and water etc to be passed through. The bed is of a heavy wood construction built into the wall and floor there is a thin plastic coated mattress on it. The toilet is stainless steel with no seat, the inside of all the doors are generally covered in scratched graffiti. the prisoners shoes and belt are removed and left on the floor outside, the doors are so heavy and self locking that when they are closed there is a resounding 'thunk'. There is a buzzer inside and a light over the outside of the cell door, if the prisoner becomes a nuisance this can be disabled from outside. The floor is also brown and smooth textured for mopping up any spills. Within the barred gate area there are interview rooms and a fingerprint / photograph room.
Hope this helps? Should you need anything further you can ask on here or PM me.
Best, Steve