A cash account inside a general investment account is different to a cash fund in the same account.
A cash fund will likely invest in money market instruments and its value (in units) may go up and down on a daily basis.
A cash account is likely to be like a bank account inside the investment account and may receive interest. This interest is paid gross and is therefore potentially taxable.
If the total interest (from all bank accounts including a cash account inside a general investment account) is below you savings interest allowances then it would be tax free, is the interest is higher than allowances it will be reportable and taxable.