Easy.
Host the forum on .com
On the .co.uk - host on all pages a link which tells visitors that the forum has moved to the .com site. This will help anyone with bookmarks find the link rather than a 404 error. Also, on each page, use the robots meta with content="noindex" so that the old content will be removed from the search engines plus they will find the new content at the new site via the link on the page.
You will do better with a wider audience if you host outside the UK, nearer your target market. However, this is of much less importance than getting international links to the site, wherever it is hosted.
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