For months, Prime Minister Keir Starmer’s Labour government has been under intense pressure from scores of its own members to demonstrate more robust support for Palestinians, especially in Gaza. But a terrorist attack at a synagogue on Thursday in Manchester, England, which led to the deaths of two people during prayer services on Yom Kippur, exposed a competing political reality for Mr. Starmer: the growing anger among Jews in Britain who believe his government is doing far too little to protect them from hate and antisemitism. The police have said the synagogue attack was carried out by Jihad al-Shamie, a...