Busbar is all you need to complete this now, it goes between the bottom "L" of the RCD, and all the MCB's at the bottom also. (See below spoiler)
Where does the 2.5 come from? This needs to be your first concern IMO. It needs to be traced and find out where it comes from/what it's connected too.
If you take the front cover off the C/U and see if there's three brown cores connected into the top of the MCB that powers it, or just two. If it's two then it would indicate that there's a switch somewhere (in the kitchen?) for the garage supply, or there's a junction box behind the wall (which I wouldn't be inclined to be the case, as it's a new build). If you take the cover off and there's three brown cores in the same MCB, I would think that one of them three cores goes straight to that outside 4x4 junction box that the SWA is connected into.