The most comprehensive and finest collections of surviving helmets from Punjab manufactured during the time of the Sikh Empire can be found in Great Britain. Surprisingly the helmets from Punjab in the National Museum in New Delhi India have not been correctly attributed to Punjab, but instead to ‘Northern India’. This leaves the helmets in various museums and collections in Great Britain as the largest repository of helmets correctly attributed to Punjab and the Sikhs, and these helmets represent the finest surviving examples of 19th century Sikh armour.
How did these helmets end up in England when they were manufactured and worn in Punjab? The end of the Sikh Empire of Lahore precipitated by the First Anglo-Sikh War of 1845–1846 followed by the Second Anglo-Sikh War of 1848–1849 and culminating in the annexation of Punjab to British dominions in India in 1849 by Lord Dalhousie, the British Governor General led to the British interest in collecting souvenirs of war, including Sikh helmets.
Although vast amounts of the Lahore Empires treasury had been looted by the Dogra Gulab Singh, the newly proclaimed Maharaja of Kashmir, Lord Dalhousie and the British Resident at Lahore Henry Lawrence had the first access to the treasures of the Sikh Kingdom and its armoury that remained. Both Dalhousie and Lawrence and their agents collected the finest examples of Sikh artifacts including helmets and armour and shipped them off to England. Once in England these helmets were distributed to the Royal Collection as well as prominent British museums and some private collections.
Lord Dalhousie himself collected exquisite examples of Sikh armour and helmets for his own private collection and these were prominently displayed at his castle in Edinburgh. The final remnants of Lord Dalhousie’s collection including helmets from Punjab were auctioned off by Sotheby's in May, 1990.  An example of Sikh armour including a helmet attributed to Maharaja Ranjit Singh’s Lahore armoury and brought to the United Kingdom by Lord Dalhousie is currently in The Royal Engineers Museum and undergoing restoration work. 
Sikh helmets were used on a number of memorial items commemorating the Anglo-Sikh Wars including military campaign medals and memorial sculptures. Sikh helmets can be seen in the representation of Sikh weapons and armour featured on the Sutlej Medal, a campaign military medal awarded to British troops following the First Anglo-Sikh War. Another Sikh helmet is also featured on the Punjab Medal issued to British troops following the Second Anglo-Sikh War.
The Sutlej Campaign Memorial at Canterbury Cathedral built in 1848 to commemorate the British soldiers of the 16th Lancers killed in the battle of Aliwal during the first Anglo-Sikh War features a Sikh helmet.
Mr. Edmund Richardson, sculptor, has completed two monuments to those of the 31st and of the 16th Lancers who fell on the Sutlej, and which are to be placed in Canterbury Cathedral. - That for the 16th Lancers, which exceeds in size the former, being 8 feet hight by 5 feet 6 inches wide, contains, in a centre panel, 4 feet by 3 feet 6 inches, an alto-relief of a wounded officer resting against a palm-tree, tended by one of his troop, who has dismounted, and with the lance in one hand, is offering with the other water from his flask, the horse standing by his side. A Sikh helmet, with gorget of chain-mail near, shows the conflict his officer has encountered. (1848) 
British soldiers also collected Sikh helmets from the battlefields of the Anglo-Sikhs Wars which they took back to England as war souvenirs:
The natives use them as well as coats of chain armour, and many of the Sikhs, as I myself witnessed, wore even breast plates and back pieces of steel. I picked up a steel helmet in the Sikh camp at the battle of Sobraon, which now serves to decorate my father's dining-room. (1854) 
The armour, like the chain mail formerly worn in Europe, was that of a Sikh, and was brought to England after the battle of Sobraon. The helmet was particularly beautiful, and had a guard to the nose such as the Norman helmets had. The crest was extremely elegant, and was made from the feathers of a water-bird which were very scarce and costly. (1866) 
1. Coulston Auction Catalog; May 21-22; 1990, Sotheby's
2. Never-displayed rare Sikh chain armour to be restored, http://news.bbc.co.uk/local/kent/hi/people_and_places/arts_and_culture/newsid_9037000/9037475.stm (September, 2010)
3. "Domestic Occurrences", The Gentleman's Magazine, July 1848, 83
4. W. W. W. Humbley, Journal of a Cavalry Officer (London: 1854), 133
5. Transactions of the Leicestershire Architectural and Archaeological Society, Volume I (Leicester: 1866), 205