The info you need is contained here
The Road Vehicles Lighting Regulations 1989
In a nutshell , there are obligatory lights and auxillary lights , all vehicles need obligatory front and rear lights eg what you call sidelights , inc taillights
When you get to Headlamps then there are different requirements ,
but with regard to landrover type vehicles, which is prob what you are interested in , then two obligatory lamps which give a main beam that can be dipped . There is then the possibility of fitting fog lights , these are called such by their usage rather than by colour or beam pattern or position (although there are certain limitations on position)
They can be illuminated at same time or as alternative to oblig head lamps mentioned earlier , as long as only used in conditions of reduced vis eg fog , falling snow. Any other light (in matched pairs) can be used with main beam , as long as you observe same requirement as main beam , eg use so as not to cause dazzle etc to other road users. They must be wired that they go out automatically on selection of dipped beam .
forgot to mention running lights these can be used with or without headlamps but are limited by light output