Your redirect is done by your Redirect line;
Redirect 301 /old-product.html http://www.newwebsite.co.uk/new-product.html
...but the file will go on processing. The last RewriteRule...
RewriteRule (.*)$ include_file.php?id=$1 [NC,L,QSA]
...will also catch the same URL and rewrite it.
What you probably want to do is to replace the Redirect
directive with a regular RewriteRule
;
RewriteCond %{REQUEST_URI} ^/old-product.html [NC]
RewriteRule ^ http://www.newwebsite.co.uk/new-product.html [R=301,L]
...which will redirect to the new URL and use the L
flag to avoid processing the rest of the file.