Using QNH within 100nm
Some of you guys are getting this bit wrong. You may use an actual QNH within 100nm of you in lieu of the area QNH, BUT all of the area QNH limitations still apply. You can not use the 100nm QNH to reduce any minimas or to waive the requirement to add 50' to the forecast QNH approach minima (Agent86's discovery aside.)
Example: You are going to aerodrome A and although it has a TAF service, there is no TAF available for your arrival. Normally you'd be required to use the area QNH and add 50' to the forecast QNH minima. However, if the area QNH covers a large area, as many do, and there is an aerodrome "B" only 40 nm away that has an AWIS, you may use the aerodrome B's QNH in lieu of the area QNH. The chances are it will be more relevant to the aerodrome you are going to than the area QNH would be. BUT you must still add the 50' to the minima, it doesn't allow you to use the TAF QNH minima or the actual QNH minima.