Sterling isn’t widely accepted for exchange into Belarusian Roubles (BYN). You should carry US dollars. Euros are also accepted.
You no longer need to present your passport for foreign currency exchange. You should only exchange foreign currency at government licensed booths. These can be found in or near major stores, hotels, banks and shopping centres. Exchanging money elsewhere can result in fines and/or arrest and the currency involved will be confiscated. It’s not possible to exchange BYN into other currencies outside Belarus, but you can do this before leaving Belarus.
There are plenty of ATMs in major cities. Credit cards are widely used and can also be used to withdraw cash.
Usually only BYN can be withdrawn from ATMs. A small number of ATMs issue US dollars but the supply is unreliable. Inform your UK bank of your intended travel to Belarus before you depart to ensure they don’t block your card because of suspicion of fraudulent use in a foreign country.