Is taking a spur to a slightly lower level possible?

[pantmawr]

Hi everyone,
I'd be massively grateful if someone could please give me some advice on this, I thought it best to ask rather than just diving in myself.
We had a new extension added to the house, plus had the wiring redone in the old part of the house (where this query is).
Ideally, we'd like to take a spur double socket to the annotated blue box (pic below) so the router can be tidied away under window seat we're fitting.
It's not a level horizontal though and would need to sit around 30cm rather than the code 45cm, but we don't have the height for that under the window.
Is there a way that this can be done? Understood any diagonal wiring is an absolute no-no,
Does anyone have any thoughts on the best approach - or if this is a non starter?
Maybe go across the horizontal level to the adjacent wall then fit a fuse for the spur, then down and along level to the lower horizontal, so the path of the cable can be clearly worked out?
We're getting a sparks in to finish off some upstairs light fittings and sockets next year, so he'd be able to check my work then - but this is just (hopefully) a mini job I was looking to sort now to get this room finished in time for Christmas.
Many thanks for any advice.

 

[Andy™]

heights can be ignored if its for a specific use. or in this case you can't put it within the recommended heights

you don't have a safe zone back to the other socket though so your cable will need protection, either steel plate or steel conduit etc

 

[SPECIAL LOCATION]

Internally looks like a pain in the bum to me!

Are there any options for going out through to external walls and accessing both new socket and existing socket positions via some holes drilled through the walls, via suitably protected external wiring?