From ARRSEpedia

The British Soldier suffers from an almost primeval urge, as soon as he (and it's usually 'he' in this case) has finished training, to discard his issued equipment and start buying some bolo gucci kit. Generally speaking, the first item on the list is a 'smock'.

Smocks come in several different varieties but, without doubt, the most popular is the 'smock, windproof' as issued to and worn by the SAS. This is distinguished by its big pockets and its hood and, in days of yore, by the fact that there was no provision for rank insignia. It's made of a cotton gaberdine which supposedly rendered it more windproof and oddly enough, until about fifteen years ago, the label inside actually described it as a 'smock, windproof, SAS' which I suppose made it easier for enemy interrogators to know who they were dealing with (this raises an interesting point: for such a covert organisation, the SAS don't half wear and carry some distinctive gear). Slightly less bolo than the SAS smock is the 'smock, windproof, arctic' which is very similar to the SAS version but has a wire hood stiffener and has always had rank tabs front and rear.

Next in popularity is the 'Para smock', the DPM version of the old Denison smock. This is a slightly controversial item: Paras regard these as a tribal item much like the maroon beret but hats tend to think of them as just another slightly warry-er combat jacket.

Widespread wearing of these items goes back to the period just after the Falklands War when a lot of British field kit had been found wanting under the conditions of the South Atlantic winter. This was compounded a couple of years later by the issue of '85' pattern combat kit that was laughably shoddy: although a better design, the materials were so bad that, for example, the big pockets would fall off if you put anything heavy in them. Oddly enough, the current issue 'Soldier 95' combat kit is easily the best field clothing the British Army has ever issued, and the issued field jacket is at least as good as any of the smocks that soldiers buy to wear in the field.