One factor is that in recent times these commercial rents have begun offering incredibly low rates for the first year, that then get increased in subsequent years. This makes it perfect for these low cost setups to move in, and move out constantly.
I do think that there is an aspect of money laundering going on. I've literally never seen anyone in one of these stores, so yeah.
Side Note: I got a weekly newsletter from Westminster council the other day, bragging about a raid they had done on some of those tourist trap places, and found £X amount of CBD products that weren't approved, non-sanctioned vapes/batteries etc. They also deported some undocumented people working there... These places are not the same as the candy places, but they are definately adjacent/similar setup.