Both surname spellings do show up in Scotland, even back to the 1200s. We Petticrews might argue that Pettigrew is a variant of Petticrew, and not the other way around.

I've seen too many instances over the centuries where a Petticrew changed his surname spelling to Pettigrew. Not so much the opposite action. Go figure.
With William Petticrew, consul-general, I venture to say that that surname spelling must have been accurate as reported, since he was a government official. Every mention of him as consular spells it as Petticrew.
Still, it is worth checking Scotland and England records for a William Pettigrew of that era, as BMD records might not have been so meticulous about the surname spelling.
Thanks for your comment.