In regards to the compliancing issue, basically it all depends on the evidence held by the RAW, so basically whatever the RAW says goes.
The ADR in relation to the brakes is ADR31, and basically it says that you can use the manufacturers brakes or brakes that have been tested to comply with ADR31, which is a very costly exercise which leaves most RAWS with only the option of using manufacturers brakes.
That takes place if your used pads are not roadworthy.