Allen, who has been working with Martin Burge at the two Michelin-starred Whatley Manor in the Cotswolds for the past seven years, said he was looking forward to put his own stamp on the menu at the 60-cover Kensington restaurant.
“This is such an amazing opportunity for me, to move to London and take over such an enduring restaurant is both thrilling and inspiring," he added.
In a statement, Des Gunawardena and David Loewi of D&D London, the restaurant's owner, said: "We are very excited that Timothy has taken the helm at Launceston Place. He is a chef of huge skill and experience, and we look forward to him making his mark at Launceston Place.”
Welch, who joined the restaurant in 2007 left his position last month to be able to spend time with his family in Sweden.
Allen is currently working on a new menu for the restaurant, known for serving seasonally changing menus based on traditional British classics. The Yorkshire born chef worked for four years for John Burton Race - at L'Ortolan and then The Landmark - before moving to Whatley Manor.