Hanging a radiator that weighs 35 kg on a plasterboard wall can be a bit tricky, but it is definitely doable with the right hardware and techniques.
You are correct that the 12mm depth of the studs might not be enough for the screws to bite. To ensure that the screws go through the whole 12mm and reach the bricks behind the studs, you can use special drywall anchors or toggle bolts. These anchors are designed to be inserted into the plasterboard and then expand behind the wall to provide a secure hold.
To install the anchors, you will need to first locate the studs in correspondence with your brackets. Then, you can use a stud finder to mark the location of the anchors on the plasterboard. Next, drill a pilot hole through the plasterboard and into the stud. Finally, insert the anchor into the hole and then tighten the screw until the anchor is flush against the plasterboard.
If you decide to fix the bracket to the bricks behind the studs, you'll need a masonry drill bit and masonry screws to secure the bracket to the wall.
It's important to keep in mind that hanging a radiator on a plasterboard wall can be challenging and it's always best to consult with a professional plumber or a handyman to ensure the installation is done correctly and safely. They will be able to advise you on the best hardware and techniques to use for your specific situation and will also be able to check if it's up to code.