Newbie basic electrical wiring question

[Horndogg81]

Hi all,
My first post on DIYNot forums.
I'm looking to replace the outside flood light on my garage with a Ring flood light current/old light shown in pic). The light is hardwired and connects via this white junction box (second pic).

The brown cable from the white junction runs down between the facia and wall into the light.
Unfortunately the earth in the brown cable has been cut short and is to short to reach the earth connection on the Ring light so I need to replace it.
Is this brown cable just 3 core flex 1.5 - or how do I tell? Does it need to be externally rated?

3 Core 3183Y Black Round Flexible Cable - 1.5mm2 - 5m | Wickes.co.uk

3 Core 3183Y Black Round Flexible Cable - 1.5mm2 - 5m

www.wickes.co.uk

Although this flex cable from Wickes is not external rated, can it be used or would someone share what type of cable I should be using for this please?
It feels silly to call out an electrician for what seems to be a simple job, although I'm happy to if needed.

Thanks all