First of all does your co.uk and .com sites both have identical content? If they do have duplicate content you have a problem that requires some work making them unique.
If not there is still no need to perform redirects. A .com address as standard means your domain will do better in the google.com index, however you can change this simply by signing up or loging into your google webmaster account.
Choose settings and select you geogrpahic targeting zone. Go here to set up your account if you don't have one.
http://www.google.com/webmasters/tools/
Once this is changed it will let Google know your preferred targeting is in the UK.
With regards to your PageRank there a lot of factors to determine. Remember if your site is still new it can take up to 9 months to be awarded PR. You need to build as many oneway links as possible and optimise your pages accordingly. Keyword Prominence is more important than keyword density. You should read a quality SEO blog like
http://www.seowizz.net/
I hope this helps