g1smd - 12:43 pm on Feb 21, 2013 (gmt 0)
You'll need a RewriteRule coded as a 301 redirect.
You'll need a preceding condition that checks that HTTP_HOST was not exactly "www.a.co.uk".
This gets the user requesting the correct URL.
You'll need a RewriteRule coded as an internal rewrite.
This will connect the external URL request to the internal folder that will deliver the content, without revealing the name of that folder.