Previous answerers hinted at the right answer; it was probably the cap badge of the Queen's Royal Lancers (QRL) you saw. Contrary to other answers, the skull and crossbones has nothing to do with the SS, Totenkopf etc - its origins long pre-date the 20th Century!
The QRL, created in 1993 following the post-Cold War reduction in the British Army, is the latest descedant of a number of former cavalry (tank) regiments.
The skull and crossbones cap badge actually originally derives (and survives) from the 18th Regiment of (Light) Dragoons.
The 18th Light Dragoons were formed in 1760 by a Colonel John Hale who had served alongside General Wolfe in the North American campaigns - Wolfe being mortally wounded whilst taking Quebec in 1759.
Hence when it came to choosing the badge for his new regiment on return to England, he choose a concept of a skull (representing Death) and the motto Or Glory out of respect to his previous commanding officer.
The 18th Dragoons were then renumbered the 17th Dragoons, being then called the 17th Lancers in 1822. The regiment took part in the infamous Charge of the Light Brigade in the Crimea and also served in the Indian Mutiny 1857, the Zulu War 1879, the Boer War 1900 and the Great War 1914-18.
In 1922 the 17th were merged with the 21st (Empress of India's) Lancers to become the 17/21st, until they became the QRL in 1993 when the 17/21st where themselves merged with the 16th/5th Queen's Royal Lancers.
The regimental badge is one of the most recognisable in the British Army.
The QRL are now a reconnaissance formation equipped with Scimitar light tanks.
The below links will provide more in depth history and current information.