Correct way to wire a security light

The boxes are normally pre-punched with 20mm holes. There's usually a few mm of the circumference which isn't punched. Drill that out with a something like a 4mm metal drill, push a screwdriver through the small hole (or use the rear of the drill bit) and bend it over to ping out the centre.

That sounds fine.

I would sink another box next to the original. You need a 10mm gap between the boxes with most common faceplate designs so they meet with no gap.

With regard to the light, fit it as high as you can, so it can cover the area by pointing downwards rather than out horizontally. Avoid spilling light outside your boundary without asking the owner of the adjacent property first, and avoid spilling light out onto a highway where you may create a risk of dazzle. After fitting, check with your neighbours that the light does not disturb them. Normally the PIR needs to be separate from the light, as their optimum positions are quite different, so combined light/PIR units are best avoided -- you end up the the light too low or the PIR too high.

Metal back boxes usually have 20mm knock outs in them. Placing a screwdriver on the edge of the knockout and tapping with a hammer is usually enough to break it free. A quick wiggle with a pair of pointed nosed pliers will usually free it.

Having said that, this is not going to help too much since you really need to be able to fuse down for the lamp first.

It would be acceptable in the sense that you have added a fused spur to power the light. You may find that you get a neater result if you were to sink another backbox beside the socket for the FCU. Or you could remove the existing socket box and widen the recess, then use a twin single backbox (electrial wholesaler), that will take both the socket and your FCU.

Aren't you supposed to label the light outside saying that its fed from a ring main?..

And good idea perhaps to have a local switched fused spur outlet so as you can flick it on and off as some of these lights can be switched to come on continuously that way....

Or drill a hole close to one edge of the cutout and screw in a self tapper, then pull that with pliers.