I suspect you haven't really got a clue what you are talking about to be fair.
The tax man doesn't have a direct insight into your bank account. Or the interest you were paid.
Also, if they do want to find out data from your bank they'll need to produce an Order to gain access to the information. They would also ask you first for it I suspect if they doubted you.
The tax office are not really interested in pissy amounts of credit interest.
Dividends probably of more interest. And blatant tax evasion.