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It seems transactions are legal but likely carefully monitored so you should be able to do it through any bank.
But there are issues, especially if the US dollar is used as that seems to be illegal.
There goes politicians naffing life up for ordinary people - as usual.

US Sanctions on Iran

Although EU/UK sanctions were lifted in January 2016, US sanctions remain in place. As a result, major banks, businesses, and individuals with any connection to the US are still at risk of prosecution by the Department of Justice if they have a ‘US element’ such as a ‘US person’ or ‘US dollar.’ These prosecutions can be civil or criminal and are mostly substantial. In the past, several major banks have been fined hundreds of millions of dollars for violating these sanction regimes. To avoid this risk, British banks have made a business decision not to deal with Iran even when the transaction is completely legal in the UK and is not connected in any way with the US.