When you buy local currency from the ATM you are exchanging currency, selling your currency and buying Takas.
Generally in countries with currency controls you should save the ATM receipts so you can convert unused local currency back to hard currency when leaving (probably with an awful rate and commission, but as you can only legally export a negligible ...
Note that the following is based on random web searches and not based in any first hand experience:
From an answer on this 6 year old thread on Trip Advisor:
... you can't buy rufiyaas outside Maldives, and you can't sell
You don't need rufiyaas at a local island either, it's enough with
This Holidify website says something ...
I have Series 2006A US$100 note. When I asked at a suburban bank branch, probably on a weekend, the employee* trying to make the line move faster told me it could no longer be used. I was surprised and disappointed.
At home, I did a quick web search and found the US Treasury web pages on US$100 bills.
USD, EUR, and GBP are the most common currencies for exchanging in Iran, so firstly you will certainly be able to exchange your GBP note. Secondly, I recommend you to exchange your note from an authorized exchange shop rather than a bank as in Iran, exchange shops always have better rates than banks.
Credit cards usually exchange at bad rates, so I'd recommend either Revolut card, or exchange cash in "Kantor". Great majority of "kantors" (exchange offices) don't take any commision and rates are competitive- much better than at banks.