the bank does not deduct any dividend amount but it charged interest which is 20%.so your question is that how bank know about my personal allowance figure of $8105? Answer is there is no any interest charged after deduction of personal allowance by the bank so it is your miss understanding,keep in mind that if you are eligible for the interest gain by investing in a bank then the amount of net interest will be 80% of your total amount because 20% income tax has been charged to you by bank of your total amount of interest.