Starling has other sources of income like their enterprise services which allow businesses to connect to the Faster Payments network: https://www.starlingbank.com/paymentservices/

In addition they have business accounts which don’t have any of the costly features like interest, free cash deposits, etc so they’re profitable for Starling as they profit off the interest (especially relevant for business accounts as they might hold high balances keeping money aside for taxes and VAT) without giving out many costly features like cash deposits (the only overhead there is support and foreign ATM withdrawals).