Repurposing old shower power

I have removed the 10.5kw electric shower from my bathroom as we have moved to theromastic shower supplied by the combi and terminated the wire

I would like to repurpose the cable for a simple LED Cabinet we are putting on the wall which needs power

I have terminated the 10mm cable (which is on its own fuse) but as I am sure you are aware it is a nightmare to bend and manipulate to reuse

From where I have terminated it (in the roof directly above where the cabinet will be)I would like to switch down to a much more managaeable cable (6mm) which I can then chase into the wall for the cabinet, leaving the the thick cable / junction in the roof where it currently resides.

Is it safe to attach a smaller cable to this terminal block which is holding the 10mm cable live, earth neutral (and its associated dedicated fuse in the RCB). My impression is that the power is drawn from the cable, rather than too much being deliveed that could blow up the LED cabinet?

Yes, but no point - just connect the cabinet to the light circuit.

Is the "roof" above a loft? If so you can terminate the 10mm into a 3A fused spur in the loft and run 1mm cable down to the light, or as above just use the existing lighting circuit instead

The cable size determines the size of the Fuse, therefore the existing fuse may be too large for a smaller cable and may need derating, or as above fuse down using a fused spur box.
If the circuit is Rcd protected another option is to fit a socket on the end and run a flex into the cuboard via a plug