Number 2 Dress
The Army's basic smart uniform, No. 2 Dress (so called because it's the same colour as baby shit) is loosely modelled on a cross between the pre-WW2 officers' and OR's Service Dress, which was actually a field uniform.
Number 2 Dress consists of a khaki tunic and trousers, worn with a khaki shirt and tie, regimental head dress and highly polished boots (or shoes if you're an officer). Most regiments and corps have nice shiny buttons (which don't have to be polished) as well as regimental collar badges (often called 'collar dogs') which are occasionally completely different to their cap badges.
Officers and WO1s generally wear either a Sam Browne or crossbelt in Number 2 Dress, whilst other ranks are normally to be seen in a green or white waist belt, often with a regimental buckle. A quick note for any Walter Mittys out there: stable belts aren't worn over Number 2 Dress tunics.