The US homebuilts can fly IFR in the USA if suitably equipped, but if they come to the UK they are subject to AWN52, which specifies day/vfr. Other countries may authorize on a different basis. For instance I've just recieved a permit authorization for Portugal which is subject to "the limitations and restrictions of the permit to fly"
It seems that they turn a blind eye to this in practice. But if something was to go wrong....