The 'Notting Hill' star gave a reading during the Fayre of St James event at a church in central London, while Lewis performed with a choir, reported Daily Mirror.
Gabrielle Aplin also sang at the event, which raised funds for the Rays of Sunshine charity, while actors Nick Frost and Tamsin Egerton gave readings along with Grant.
The concert was followed by a Christmas party featuring a silent auction to bring in funds for the charity, which grants wishes for terminally-ill children.