Great Britain has laws allowing arrest of war criminals. Could Obama enter the country or there is diplomatic immunity?
US may be guilty of war crimes over drone use – Amnesty Intl
And you are wrong again as the UK has no such laws. What we do have is a system where a private individual can enter a case of war crimes into civil court and the person named can be made to face the court and explain their actions. The court will then let them leave as it decides what action to take.
Unless there is an INTERNATIONAL ARREST WARRANT out for him, which is unlikely as being a statesman he is immune from such things.
Guess you attack on the great shaitan has fallen on its face