Hi Joanne,
When taking such a decision I always read through again and again the article from Google: https://support.google.com/webmasters/bin/answer.py?hl=en&answer=83105
Ideally it would be good to contact each webmaster and ask them to change the url from the .com to the .co.uk, but realistic that is almost impossible.
What I would do is download through OpenSiteExplorer all the incoming links, check in general which are the most important for me and at least contact those webmasters and tell them about the change of address.
I hope it helps.
Istvan