How to manage wiring if replacing ceiling rose with anchor for heavier light
I cannot believe that the ceramic ceiling connection block is to be removed unless it would be replaced with another one.

I would expect that the metal bar is to be held with screws through some of the pairs of holes in the ceramic block penetrating into the wood structure above. I see what looks like breakaway covers over extra holes just outside the ones with the brass wood screws. What is the distance between those holes? What is the distance between the slots in the metal bar? Are these the same?

EDIT

Yes, I am referring to the two on the same line as the holes with the brass wood screws with are being used to hold the connection block. I believe that the projections on the ends of the bar go up toward the ceiling. If the slots in the mounting bar are the same distance as the inner pair of holes, you could break away the covers on the outer holes and shift the brass screws to those outer holes. Then use the inner holes to secure the mounting strap. If there is not enough clearance for the wires (with the projections on the bar projecting up), you'd have to get some spacers.

EDIT2

Another possibility would be to drill two new holes further out in the mounting bar. You should find out how this type of ceiling fixture is normally installed in the UK. This is not something to try to puzzle out on your own.

EDIT

Looks like I was completely wrong! See this video for installation in the UK. Video

EDIT

It could be that one could remove the ceiling rosette temporarily to place the bar against the ceiling (end tabs down of course), then replace the rosette on top of the bar. The fact that the slots in the bar would line up with the holes in the rosette suggests that this was intended as a possibility. This would allow the use of the connection block instead of getting a bunch of individual connectors for the various wires. I don't know if the UK fixtures have enough clearance under the shroud for the rosette and whether this is a practice in the UK. To do this one would place the bar on the ceiling so that the mounting slots line up with the existing holes in the ceiling then mark the countersunk hole for drilling a pilot hole and secure the bar to the ceiling with a single screw in the countersunk hole in the bar, then secure the rosette over the bar through the lined up holes of rosette and bar.