Yes everything is taxable Cash, Cheques and Bank Transfer etc. Your Accountant would ask to see your Receipt Book as you suppose to give a Receipt for payments receive from Domestic Customers as for Invoice this is the proof of income cominng into your business.Your Cash Book should have Cash In on one side and Payments on the other side so if you brought a sandwich you can prove that you spent a bit of income coming into the business so everything is accountable.