'...I've left it parked in the motorcycle bay outside of my flat chained without any issue.'
What 'bay'? The two photos don't show any traffic sign designating an area as a parking place and, even if there was, what's it's lawful extent?
Below is the law and where it refers to a 'resolution', this includes a designated parking place properly signed.
4)A highway authority may by resolution, or in the case of the Secretary of State by such notice as appears to him to be appropriate, authorise, from a date specified in the resolution or notice, the parking of vehicles[F7—
(a)]on, or [F8on or over a road or part of a road] which is a highway other than a carriageway[F9; or
(b)on or over a footpath or any part of a footpath];
and notwithstanding the provisions of this section or of any other enactment or any rule of law, it shall be lawful from the specified date for any person to park a vehicle on the [F10road or footpath], or on the part thereof, as the case may be, to which the said resolution or notice relates and is for the time being in force.
IMO, you seem to be assuming something which has yet to be established.
Clearer and wider photos pl.