Interchange is the fee your customer's bank keeps. For consumer cards inside the UK it is capped, which means nobody can quote you a higher number for it and call it their price.
0.2% of the transaction on consumer debit cards, including prepaid. 0.3% on consumer credit cards. The caps apply when the merchant, the acquirer and the card issuer are all in the UK.
Split last month's card turnover into UK consumer debit, UK consumer credit, commercial and cross-border. Apply 0.2% and 0.3% to the first two. The rest of your bill has nothing to do with interchange.