Everything looks in order with your wiring. Don't move your modem upstairs with your current setup whatever you do - with the MK3 plate in place the modem should only be plugged where it is now.
I think you should decide on a long-term setup and stick to it. Your wiring looks fine and I'm sure the engineer would have used it to make the study the master socket. For yourself, probably best to make the study socket a 'modem socket' and the existing wiring to it a 'modem cable' and keep the modem upstairs only. Simply move the blue wires currently terminated at the faceplate to the two terminals on the MK3 plate (you should cut off the ends of the wire and terminate afresh with an IDC tool) and swap the phone socket in the study for a RJ11 socket.
You should note some off-peak speed tests and ping tests before, then after, and then a while after, to monitor any adverse impact. If you want to buy a HG612 modem these can be easily unlocked in 3 mins and you can see the line stats. Or if you want a VDSL modem/router you could buy that first.
Edited by deleted (Thu 20-Aug-15 20:57:08)