OK
There are a number of issues, if you have used an isolator, this is merely a switch, and provides no protection to the cable.
This cable must be provided with both overload and fault protection, usually via a fuse switch (assuming swa all the way).
One way of back-checking your work, is to imagine what an experienced person would assess the work both as an initial installation and as an eicr. If you did this it would attract a C2 for both missing fault and for missing overload protection (2×C2).
I still don't quite understand the boxed in part, if this is proper trunking then that's one thing, however if not and it's a matter of being behind metal/plastic/timber covering that looks architectural then there are further issues, firstly as the cable has only a single layer of insulation this would attract a C2 - a second layer must be applied, a colouring sheath would not meet the requirements as it should not be removable without tools/destruction and is generally not of the required thickness/insulation.
In addition, now this cable is hidden less than 50mm in the building structure, rcd protection is required, some would code this as a C2 some C3, I would normally consider it C3, however as a substantial supply to a sub-main I would give this a C2 - driving a nail into a 1mm^2 - 2.5mm^2 cable protected by a 6A/32A mcb is one thing, into a 16mm^2 unprotected cable is another!
If it's a matter of a few ft, can you not move the garage cu? Or install proper steel conduit/trunking