On a calculator have this calculation in mind. Interest receivable tax allowance divided by the interest rate you're getting. This tells you the maximum you should have in your savings accounts that aren't sheltered from tax.
Example: You have funds in an account that pays 3.8% interest. Your allowance of interest earned before paying tax is £1,000 and on a calculator you express a % as a fraction by dividing it by a hundred if you're not comfortable using a % button. So in this example 1000 / 0.038 = £26,318, that's the maximum you should keep in those regular savings accounts. Any more and you'll pay tax on additional interest received beyond £1,000
Of course it isn't quite that simple because any interest you earn is added to your savings every month most likely, the interest rates will not remain static for the whole year (April - April) and it's harder to be precise if you have it in multiple accounts paying different interest levels. But it is an easy bit of math to understand if you're going to be close to the limit of needing to pay tax.
Unless there's a big disparity between interest received between non-ISA and ISA options, as can be the case, in general maxing out your ISA allowance is just a smart thing to do - it's protected from tax now and forever more. Although not quite as straight forward as moving money between bank accounts, it's still perfectly possible to move ISAs around between providers annually (or whatever period you lock in) in future years to get better deals too.