Interest rate vs access – most easy access accounts offer a lower interest rate than other savings accounts, while the higher interest accounts typically have restrictions on when you can access your money.
Minimum and maximum deposit  – some accounts can be opened with just £1, while others may need you to deposit much larger sums. Some savings accounts will also have a maximum balance. A number of accounts offer tiered rates of interest, so the amount of money you have in the account can affect your rate of interest.
Tax – an ISA should help you maximise your tax-free earnings.
Protection  – make sure your savings are protected by the Financial Services Compensation Scheme (FSCS), so you know you'll get most of, or all of your money back, if the organisation fails.
If you've only got a small amount to save, some current accounts will pay higher rates of interest on balances up to a certain level, so these may be worth considering over savings accounts. But you'll need to be disciplined and make sure you don't spend the cash in your account. It might be worth opening a savings account, too.