SET UP A SEPARATE BANK ACCOUNT FOR YOUR TAX
(preferably a Savings Account so you can get some interest)
It is suggested for your Self Assessment Tax that you put in 30% of turnover (total sales) if you are a Sole Trader.
For a Limited Company take 19% of turnover for Corporation Tax.
If you are VAT registered you need to consider taking a further 20% of your turnover (total sales) after deducting 20% of your purchases that have VAT charged upon them.
With a good and up-to-date accounting system these estimates should be easily calculated.
You will then have enough money available to pay your HMRC tax liabilities when due and will not suffer cashflow problems.