I have assessed this question with a focus on British English because your quotation is from the British Prime Minister.
I did initially think you had either misheard the word rounding, or that Prime Minister Boris Johnson from whom you were quoting had made a characteristic gaffe:
We are committed to rounding up the evil county lines drugs gangs.
This is a common expression and would make a lot more sense. "Rounding up" idiomatically means to gather in or bring together. It was historically used to describe the herding of cattle but now is commonly used to describe catching criminals.
"Roll up" has several common idiomatic uses but none that would fit this context. However, it has been pointed out to me that this expression is sometimes used to refer to catching criminals in a similar way to the expression "rounding up".
I do not believe it is widely used in British English as I have never heard it before now, and have only been shown a small handful of news headlines using it and as yet have not seen this definition in any British English dictionary. I suspect that it is a phrase "borrowed" from American English.
So it would seem that the quotation means to catch, get rid of, or in a sense "defeat" the drugs gangs.