Iains, which bit of "Not the only effect but an effect" did you not comprehend?
I think he was pointing out that Brexit was not the sole cause, of the effect.
Saying "Not the only effect but an effect" still suggests that all the shop closures were caused by Brexit, and that Brexit has caused other effects as well. It does not clarify that Brexit was not thye sole cause of the closures.

"Cause and effect" are often linked, but are not interchangeable.