The UK isn't threatening to bring him up on charges. British law states that even a private citizen can bring someone up on charges of war crimes. Smart man, he knows Britain has a large Muslim population, and any one of them could yell "War crimes!!" The government would then be obligated to follow through.
At least 1,000 Israeli Arabs protest death of Palestinian infant in deadly West Bank arson (Haaretz)
from the article: London's burning for Dichter