Every country in Europe operates a reciprocal concession whereby vehicles which are legal in their country of registration are allowed to visit, for up to six months, another country without complying to vehicle regulations.
Driver and road regulations have to be complied with, even by visitors.
In the case of the UK, a stay longer than six months would require the vehicle to be registered here, at which point it has to comply with our regulations.
So we do have this anomaly, that we can only tow 2.5m trailers with a towcar exceeding 3500 MAW while visitors can tow such trailers with smaller vehicles.