Can't list tax as an expense...

Quite new to receiving UC, Self employed - Income fluctuates. I have always set aside 20% of earnings and put them into a pot in my business bank to prepare for Self Assessment.

I have just been told that I am not allowed to do this and it isn't classed as an expense until they can see it leaving my bank and going to HMRC. So yes when I actually submit my tax payment I'll probably have a good month for UC but along the way they will think I have approximately £400 more in wages for the month... But I don't. The money is stored. This will leave me stretched monthly. Do I stop preparing for tax and take the additional income. So worried.