Continental has merged into United, so I presumed that since Continental is still on the list, United is by default also approved.
I actually sat on hold with United for over an hour, and in the end had to hang up, so I might be flying my dog on Delta just by default!
The one thing that makes me feel better about this is that I could be on the same flight, a direct flight straight into LHR, whereas with United, there'd be at least 1, probably 2 layovers - and I'm a little concerned the extra transfers would stress him out, or allow room for something to go wrong (missed connection, getting loose, etc)
Or is this just wishful thinking??