Changed an old Led light to pendant and it keeps shorting
You shouldn't need all those floating connector blocks, there are enough positions on the backplate for loop-in, loop-out, switch and pendant.

The pendant should be connected to the outermost positions and looped through the cord grips.

Your wires to the switch probably include a red permanent-live and a black switched live which should be marked with red tape and connected at the double position connector.

  

In the above diagram, A is loop-in from prior ceiling-rose (or from consumer unit), B is loop-out to next ceiling rose and C is to/from switch. Typically all wires are 1mm² or 1.5mm² twin &amp; earth with green/yellow sleeving added to the bare earth wires.

Note that I think most UK electricians wire the wires from the switch cable (C) in the other way than that shown in this diagram, they connect the red to position 3 and the black (with red tape) to position 2. That's the way I, as a mere home-owner, wire these fittings.


Photo - own work, &copy; RedGrittyBrick. Licence: Creative-Commons Attribution-ShareAlike 3.0

At the top are three 1.5 mm² Twin&amp;Earth cables with the 2004 colours (blue neutral, brown line/live) the bare circuit protective conductor (CPC or Earth) in the cable is covered by a green/yellow sheath when terminated.

From left to right these are feed-in, feed-out and switch. The blue from the switch is covered in red tape (probably should be brown?) to indicate switched-live.

The flex to the pendant lamp is looped around the strain relief.



Could it be that in your case, the black switched-live is not marked with red tape and has been connected with the other blacks - this would short the circuit when the switch is turned on