When you search the Commonwealth War Graves Commission website for the death of W. Lamb you get a list of 72 names. Some of these can be easily excluded but none have any clear connection to this part of Sussex or neighbouring Hampshire. There is no trace of a William Lamb or any Lamb family in the 1911 Census records for Iping and Chithurst.
Here are brief biographical notes on the three individuals who seem most likely to have any connection with the parish of Iping.
Private William Lamb (service number 25022, 2nd Battalion Hampshire Regiment)
William Lamb was born in 1885 in the district of Romsey in Hampshire, the son of William Edward Lamb and his wife Elizabeth a cattle drover and later dealer. The family lived in the Romsey area at least until 1911 although at different addresses. He enlisted into the 2nd Battalion of the Hampshire Regiment at Winchester and died on 19 August 1918. He was buried at the Borre British Cemetery in Northern France.
Gunner William Lamb (service number 216198, Royal Field Artillery X 55th TM Battery)
This man was born in Corton Denham near the border between Somerset and Dorset between Wincanton and Sherborne in 1895. He was the son of Ernest Lamb an agricultural labourer and his wife Sarah. The family continued to live in the village and in 1911 William Lamb was working as a stable boy. He enlisted at Bordon in Hampshire (which is only about five miles from the parish of Iping) and joined the Royal Field Artillery. He died of wounds on 7 November 1918 at the very end of the war and was buried at Froidmont Communal Cemetery.
Private William James Lamb (service number 5078, Royal Berkshire Regiment 1/4th Battalion)
William James Lamb was born in 1898 in the district of Hartley Wintney, Hampshire, the son of George Lamb a cowman and his wife Ellen (or Helen). The family moved north to Berkshire and by 1901 they were living in Waltham St Lawrence, before moving to Hurst near Twyford and later to Binfield. William and his elder brother Isaac both enlisted in Reading into the Royal Berkshire Regiment although probably at different times. Isaac was killed in November 1914 and William James in May 1918. He is buried at the Gezaincourt Communal Cemetery.