It is easier to come to fair common terms for two parties when both are strong (i.e. both have nuclear arsenal US-Russia, US-China), rather than when one is weak and the other is strong. For example, US, UK have been bullying weak nations for generations, while to a more extend offering fair solutions in case of Russia, India, and China disagreements. Either Iran should too acquire nuclear arsenal, or US and UK should dismantle theirs.
Iran's Khamenei bans all further negotiations with U.S. (Reuters)
from the article: Israel attack wouldn't stop Iran nuclear program, says U.K. study