Having seen the photo I assume that the meter box is going where the block is you may just have enough to make off in Henly blocks but you have to consider that on the LHS is normally where the cut-out is. If not they will have to be replaced and the developers will have to get to grips with that is what will have to be done you should not be having to bodge a new installation.
I disagree with your last posted comment,recognised methods for extending cables may not be to our personal taste but are fully compliant
I disagree also with your earlier comment which I have highlighted above f
The entry of the tails into the cabinet should be upwards from the bottom
Cables entering from above can cause any damp or water ingress to run down the cable into the joint or equipment where they terminate