There's a couple of different ways you can do your 301 redirect. You can do it directly in your IIS configuration. You'd need to setup a pattern redirect to redirect all the .com to the .co.uk domain.
You can also use a software based redirect, using a DNN module to redirect the particular pages. I sell a product in this area, in which you can enter a setting to redirect all Urls from one set portal alias to another (effectively redirect all yoursite.com to yoursite.co.uk, plus cleaning up any www / non www mixups as well).
Unless you've entered full Urls into your html code (including the domain name instead of a relative ur), you shouldn't need to update your html. Besides, with a 301 redirect solution, the html links will still work, but it's a good idea to clean them up over time.
With regards to your domain extension, it makes good sense to pick the market in which you're targeting. The .co.uk will likely give you a boost in the uk market but not in the us or international market. The original links aren't that important because the search engines will follow the redirects.
If you're interested, you can download a trial version of the software at http://www.ifinity.com.au/Products/Url_Master_DNN_SEO_Urls/ - when you do, request a trial licence for your site and I'll give you the information on how to setup the portal alias redirect (it's not in the included documentation, as it's a feature designed to rescue these sorts of situations).
-Bruce