.com vs .co.uk site. Penalized by google for duplicate content?

If you make a new content but still relevant to the old site then you would be fine.

Alternatively, you can shut down the old site.

Use cross domain rel-canonical tags. Problem Solved.

Official Google Webmaster Central Blog: Handling legitimate cross-domain content duplication

I would just ditch one of the domains if you don't have a strong reason for it.

It won't be seen as duplicate content unless you use the same product descriptions on both sites. Just rewrite the product descriptions and you'll have nothing to worry about.

Canonical's are a hack, they can work or cause more harm than good. The correct way is to structure the site correctly without them. A country specific tld needs country specific content (think Sephora) and therefore is double the work to run. Will there be duplicate content issues, likely not but there are so many factors such as local content, pricing in local currency, social media efforts, etc. that it actually becomes impossible to answer with clarity. Google works on an indirect method, if the sum of the parts do not match you will be penalised - each sum is unique to a site. The top international companies use single-tld for all countries, but no SME software can do that with international pricing - except, we use a layer over Magento that can but it was designed by architects who work for the largest companies in the world. We have done same description multi-country multi-tld sites with no issues - same description multi-country single-tld sites with no issues - but we structure the content like the multi-nationals so Google doesn't object. Basically you should be 80% in the clear, but if you do anything too stupid Google will throw the book at you.