Accessing cash in Iran is difficult. Present currency volatility is making it even more so. Non-Iranian debit and cash cards will not work in Iran. No cash machines accept UK bank cards. It’s usually not possible to change travellers’ cheques.
It’s illegal to change money on the street. You should be clear before travelling how you will obtain enough rials for your visit legally, and, if unsure, you may need to reconsider your travel plans.