Hi Keith,
In my experience Google mainly crawls from the US.
You're quite right, GeoIP redirection can cause major issues with indexation - as if you're redirecting everything from the US to the .com googlebot can't see the .co.uk site.
As such, I'm not a fan. Rather than implementing a hard redirect I prefer Amazon's approach. If you visit the amazon.com site from a UK IP you get a javascript overlay that invites you to visit the .co.uk version of the site instead - they let the user decide which site to view rather than actually redirecting them.
This is a nice solution as it ensures that the search bots can crawl both versions of the site, and rankings aren't endangered.
I hope this helps,
Hannah