Once you're paid for a job, if you're paid gross (without tax taken off) then it's good practice to save the amount you'll need to pay in tax and national insurance in a separate account,  as it's not part of the money you have available to spend, whether that's on business expenses, or to pay yourself a 'salary'.
You're also then not scrabbling around for the cash when the tax falls due. Usually you pay what you owe in January and half in July, though due to the coronavirus, HMRC's made an exception this year, and there's no payment due in July, though you will still be liable for the entire tax bill.
As a rule of thumb, if you're a basic-rate taxpayer, you should be putting around 30% of your profit aside for tax and national insurance, or around 40% if you're paying higher-rate tax (use our income tax calculator or check your tax rates). Your profit's worked out as your income minus your expenses.
All the fintech business accounts above let you easily set up a separate 'pot' for your tax bill cash, where you can transfer money in as you go. If you've chosen the high-street bank route, you should be able to set up a separate account for tax and move money across, though often this will be a lot more of a manual process.
If you're a director of your own limited company, your tax affairs will be a little more complex as you'll often be paying corporation tax rather than income tax. How it works for you in practice will depend on whether you're using an umbrella company or you're directly contracting.
Regardless of your company set-up, if you're not a numbers whizz, or you're time-poor, a good accountant can be worth their weight in gold, especially when it comes to self-assessment time.