The law may well differ across member states. Apparently there isn't a definitive law on the matter in the UK, but you could be prosecuted if your roof load is

The law may well differ across member states. Apparently there isn't a definitive law on the matter in the UK, but you could be prosecuted if your roof load is deemed to be dangerous or is excessively heavy or overhangs the car. There definitely isn't explicit rules that require roof loads to be checked or qualified in any way. It's all moot though as they did it in Germany...