This is because you only need to declare money when it comes in and out of your business. At the end of the tax year, you won’t have to pay Income Tax on money you didn’t receive in your accounting period.
When cash basis might not suit your business
Cash basis probably won’t suit you if you:
want to claim interest or bank charges of more than £500 as an expense
run a business that’s more complex, for example you have high levels of stock
need to get finance for your business - a bank could ask to see accounts drawn up using traditional accounting to see what you owe and are due before agreeing a loan
have losses that you want to offset against other taxable income (‘sideways loss relief’)