It is illegal to cycle on a public road after dark without lights and reflectors. Exactly which lights and reflectors, where to fit them and when to light up, is defined by the Road Vehicles Lighting Regulations.
When you cycle into another country, your 'temporarily imported' bike doesn't have to be exactly like the bikes of that country, provided it conforms with a very basic, internationally agreed specification.
Cycling UK is a trading name of Cyclists’ Touring Club (CTC) a company limited by guarantee, registered in England no: 25185. Registered as a charity in England and Wales charity no: 1147607 and in Scotland charity no: sco42541. Registered office: Parklands, Railton Road, Guildford, Surrey GU2 9JX.