The health ministry said the cases with the UK variant were two people from India and one from China, all of whom had been isolated.
The B1.1.7 variant, first found in Britain, is highly transmissible and its discovery prompted a tightening of travel restrictions globally among countries keen to keep it at bay.
The charges against party leader Kem Sokha stem from accusations he conspired with the United States to overthrow Hun Sen - accusations he and Washington reject.