Andy, firstly - you don't descend to circuit height in the circuit not unless instructed by the tower ( e.g. join downwind). If you intend to land at an uncontrolled aerodrome you descend to circuit height on the "dead side" of the circuit. The "dead side" is the otherside of the circuit. Eg. If the runway has a left hand circuit as your drawing indicates, you descend to circuit height on the righthand side of final and upwind and when clear of traffic, you join crosswind or mid crosswind. I have never done an "Upwind Join". I'm not sure if anyone has not unless you are doing a go around I guess. Personally I wouldn't do it just in case I missed seeing an aircraft joining crosswind or taking off (upwind). If the tower instructs you to, then so be it. I would like to think that if this is the tower's instruction, they have already taken into account the traffic situation.
Hope this helps
D6