Dill, are you registered? Your advice is from the 70's, and whereas we do all have our own ways, there are right ways and wrong ways. Yours is not technically "wrong"(although it could be argued that you are introducing unnecessary resistances), just not right.
Why do you feel a second valve is required?
OP: An external cock is NOT required. It is, nowadays, pretty pointless. Adds costs to a job needlessly.If visible they are ugly, and to chase out a wall and have a skirting prepared so it is easily removable is finding a solution to a problem that you have just created. Every fire that I have seen for many years has a built on isolation valve.
Make sure your installer is Gas Safe Registered, and has Gas Fires on his card - many do not. Importantly, make sure he has EXPERIENCE in fire place fitting.
Before doing any prepping, select an installer.Have him do a survey (be prepared to pay particularly of you do not proceed), and get his advice.
You do NOT need visible pipework, valves or mantle hanging brackets