Although many banking restrictions against Iran were lifted in January as part of the nuclear agreement reached last year, UK banks certainly aren't rushing to open shop in the Islamic Republic. 

The reason is that whilst many sanctions were lifted, certain U.S. sanctions against Iran related to missile proliferation and support for terrorism remain in place, with the U.S. recently announcing that it does not intend to grant Iran access to the U.S. financial system. 

The U.S. has emphasised that it is not seeking to deter foreign banks from dealing with Iran, going as far as offering clarity around which activities are permitted and which aren't. Yet for banks with U.S. operations (which is virtually all UK banks), the risks are clearly too high - lets not forget that Standard Chartered was fined close to $1b last year for Iran-related dealings.