So when british buy in poland and then sell in poland they are in fact, helping the economy
It depends.
It reminds me of a joke a Jewish friend of mine told me:
Itzhak accepted a bet of $100 that he'll pay Moshe if he eats a pile of crap. The bet went through, the crap was eaten the $100 changed hands. Now, the Itzhak who lost the $100 felt a little poorer so they made a bet again, but this time the Moshe was to pay up if the Itzhak ate crap too. Again, the bet worked and $100 went back to its original owner.
Struck by a moment of reflection, Itzhak remarked, "you know Moshe, we both ate a pile of crap each and in the end we are where we were without eating it."
"I know", replied Moshe. "But at least we kept the economy going".