You can use a low Bb bari for rock. One of the most popular session players in LA, David Woodford, uses his low Bb Selmer Mk VI with a low A extension that Paul Coats devised. Here's a link to how to make it:
http://www.bassic-sax.ca/low_a.html
Having said that, I have both a low Bb and a low A bari. I like both of them, but for different reasons. I tend to use my low Bb for jazz and my low A for rock/blues...But that has more to do with what each sounds like than anything else. I've got a Selmer VI low Bb, and a Medusa low A. The Medusa has a much edgier tone and lends itself better to the blues band that I play in. However, before I got the Medusa, I always used the Selmer, and that held it's own in a rock/blues environment.
Play the Keilworth. See if you like it. Compare it to the horns you've played in the past. The best comparison would be of course be to compare a low Bb and a low A side by side and pick the one you like better.
Either way, don't let the perceived need for a low A determine the horn you get. Pick the one that you like the sound of better. (Assuming that the horns are close in action, set up, quality, etc. etc.)
