Seems the museum is not happy about the incident either. Since museums like this one are large organizations, staffed by a core of paid professionals, but a lot of the leg work done by an army of volunteers, someone did a "loose cannon" act here. I have not found a follow up article, but it appears the museum wants to restore the image to its original form with the cigar. (article link in blue below)
Churchill Museum Picture Missing Cigar
Uniform, victory salute and cigar: Winston Churchill in the 1940s and now without his trademark smoke.
The Britain At War Experience in South-East London with the airbrushed picture of Churchill above the entrance
Dog Soldier
Fast is fine, but accuracy is everything.
- Wyatt Earp