I don't have a lot of knowledge about how effective redirects are, but(btw, bought domain yesterday) my first thoughts are that it would be more effective as a live content site. That being said, first thought are often wrong.
If I wanted to risk temporarily losing my traffic, I would move the content to the .co.uk domain.
Right now, I'm #1 on Live Search and about page infinity on Google. MSN/Live traffic is about 80% and Google 15%.
I think the shift would bring me considerably higher on Google.uk since my UK traffic is easily 90% of total.
As you suggested, I could keep both sites live and just do a .htaccess redirect, but I have heard that SE's, especially Google, really don't like that. And how would MSN/Live react to the redirect.
One option I'm considering is making the new site an extensive FAQ.
I'm hoping someone a lot smarter than me can fill in some of the blanks.