I realise that the SAS was formed in WW2 - but were their no similar groups that were actively behind the enemy lines, attacking from behind, or destroying roads, supply routes, and such things?
The simple answer is this: the concept of modern commando groups was borne out of WW2, two decades after WW1.
Special Forces: A Unique National Asset by Mark D. Boyatt covers the history of US special operations forces, and covers a bit on the historic roots of special forces around the world, and it's quite clear that special forces - the kind you are imagining like the SAS operating deep behind enemy lines - is an invention from WW2.
Keep in mind that logistics are a big reason for this: before the invention of airborne infantry or reliable motorized transport, getting behind enemy lines and staying there would have been difficult to impossible.
You might want to consider searching about the Stormtroopers, the Ardetti, and various other trench raiders.
These units would sneak across no man's land and attack isolated outposts to capture prisoners, seize weapons, or destroy important defensive positions. They practiced small unit infiltration during larger offensives, working to clear trenches so that reinforcements could exploit the gap. Trench raiders can be considered a forerunner of WWII era special forces