An FCDO spokesperson said:
The UK condemns in the strongest terms Iran’s execution of Arman Abdolali, after he was sentenced to death for a crime committed when he was a minor.
Iran has ratified both the United Nations Convention on the Rights of a Child and the International Covenant on Civil and Political Rights. Both prohibit the execution of minors at the time of the offence. His execution is therefore a wholly unacceptable breach of international law.
The British Government is firmly opposed to the death penalty in all circumstances and in every country, especially in cases that do not meet the minimum standards defined by the International Covenant on Civil and Political Rights. We call on Iran to refrain from any future executions.