To all intents and purposes an ATPL will permit you to become a Captain with any outfit!
Not in mine. Not in BA nor in most of the big players that I know of. Just the wee small matter of seniority, potentially a 10 year or more wait and meeting the minimum hours set out in your OPS manual which could well be way above the 1500hrs required for an ATPL. Most candidates would have 2 or 3 times the minimum hours in our airline due to the long wait.