Either a 12"x12" or an 8"x16" sheet of 14 gauge steel or 10 gauge aluminum. Also a similar sized piece of at least 1/2" plywood.
Use masking tape to cordon off an area of the table 1/4" on each edge smaller than the sheet of steel. With a 120 grit block sander, sand up that area so the surface is rough. Do the same thing on the underside, and also on the sheet of plywood.
On the steel or aluminum, identify a "pretty side" and flip it over. On the un-pretty side, roughen the surface - lots of microscopic hills and valleys - using #36 sandpaper, light touch with a grinding wheel, whatever you've got.
Peel the original masking tape and re-mask an area just exactly the size of the sheet of metal. Mix up the slowest epoxy that the hardware store has, and coat the rough side of the sheet of steel and the desk surface. Set the steel there. Do the same thing with the plywood on the bottom, then clamp with wood clamps. We want there to be no voids between steel, desk, desk and plywood. The point of gluing is to assure that pressure is spread.
When the epoxy gets rubbery but before it cures hard, use an exacto knife to cut away any excess and peel off the masking tape. If you wait til hard cure, you'll never get it up.
Remove clamps. Mount up your stand!
Why are we using metal up top and plywood below? Aesthetics. Plywood up top would be fine if you wanted a huge bump on your desk.
Why a brutish #14Fe/#10Al sheet (Humvee frame) instead of a pie tin? To spread lateral force instead of dent. If you were capable of bending it without a metal brake, then so is your monitor clamp, which means it will still inflict point forces on the desk surface and will still collapse it!
Why so darn BIG? Because you need to engage quite a large area given the finite compression per square inch of the cardboard honeycomb. We also need to span quite a ways away from the damaged area to get enough good material to do the job.
Why not just repair the damaged area? Because that's a bunch of fidgety dental work, and I'm guessing you already think the above is too much work.
Won't the steel, epoxy and plywood cost more than the whole £29 table top? Why yes, yes it will. The question is whether it'll cost more than a table top made to clamp monitor stands!