Well not exactly true; you can use Arsenic instead of Tin and there is evidence that they certainly did use it; I am not sure of the breakdown of ancient tin-Bronze vs arsenic Bronze in use. In any case your point still stands as we also know there was tin bronze in use and likely much of it may have come from the British isles.

Would arsenic bronze have any added toxic effects?

More to the people producing it than using it. But metallurgy and mining must have been awfully dangerous industries to be in back then regardless of the arsenic.

I was thinking more along the lines of "added weapon poison damage", ex. https://worldbuilding.stackexchange.com/questions/99646/can-...

Seems that lethal dose for arsenic is likely too high to be transmitted by even a pure arsenic trioxide blade, unless it was manufactured with the intent of leaving bits inside the wound, like maybe a blade a rough brittle surface to leave grit. Even so, would have to introduce ~50+mgs worth, which sees like a tall order.

Seems arsenic bronze is even less toxic, so probably not poisoning anyone. I wonder if the minor local toxicity of arsenic blade cuts would reduce the chance of the wound causing sepsis.

Perhaps an arsenic bronze arrow head broken off inside of somebody could poison them slowly over time.