I assume these are two separate tickets, right? (Does AR even interline with UA?)

If that's the case, and even if it's not, I would really not do this. It's very likely you're going to find yourself stuck in BA after a delayed arrival.

When I used to go to Patagonia for work, I would fly back to BA on a morning flight the day before my onward departure to the US. On one occasion, my early morning flight to BA became a late afternoon flight with no advance notice.

There are far too many elements outside your control to risk this connection, in my opinion.