Assuming you mean the pre-20th century, remember that many of the technologies that make modern special forces effective - air craft, stealth equipment, portable heavy firepower - did not exist. The British couldn't exactly helicopter an SAS team to the roof of Independence Hall.

That being said, there were some units that selected men for their physical ability and martial prowess, and these units were given tasks exceeding what normal infantry were called upon to do. During the American Revolution, British infantry regiments contained one company each of grenadiers (big, strong men) and light infantry (fast, nimble men that were noted to be good shots). Membership in these companies depended on nomination of an officer, and generally required soldiers to be well behaved in addition to being militarily proficient. The light infantry was generally tasked with skirmishing and flanking, while the grenadiers usually made up the heart of traditional attacks and bayonet charges. Matthew Spring's With Zeal and With Bayonets Only examines these two specializations in detail, and covers the British Army during the Revolution generally.

Beyond that, certain line regiments were consistently held in better regard than others. The Coldstream Guards (2nd Regiment of Foot) was a showcase regiment. The 33rd Regiment of Foot were consistently noted for their excellence, and obtaining the colonecy of the regiment was a frequent fast-track to making general. Case in point: Colonels of the 33rd include the Earl of Cornwallis and Arthur Wellesely, later the Duke of Wellington (after the Duke's death, the Regiment was named after him). The Royal Welch/Welsh Fusiliers (23rd Foot) were also noted for their general excellence. The two regiments were brigaded together during Cornwallis' campaigns in the South, forming the more-or-less unbeaten right wing of his army. Check out the memoirs of Serjeant Roger Lamb and Mark Urban's Fusiliers for more on the RWF and frequent mentions of their partners in the 23rd.

During the Napoleonic Wars, the Rifle Brigade was an excellent corp of chosen men that saw dramatic innovations in both tactics and military discipline. Urban has a book on them, as well, called Wellington's Riflemen.

Can you clarify what time period you are looking at?

The British Empire spans the eras of horse and musket to the era of nuclear weapons and jet aircraft, and arguably still exists in a very much reduced form.