g1smd - 12:59 pm on Feb 25, 2013 (gmt 0)
Add a blank line after each Rule so you can see what code is processed together. Conditions apply only to the single following Rule.
As coded, a request for b.com/<anything> with or without www is redirected to a.co.uk/THISPAGE1
The new request for anyhostname.tld/THISPAGE1 is then served by the internal script.
Every rule needs the [L] flag.