Personally I would use the company name so
"Big red widgets from Widgetco plc"
not
"Big red widgets from widgetco.co.uk"

Personally, I like:

<title>domainname.co.uk - big red widgets available in small and medium</title>

or:

<title>Domainname - big red widgets available in small and medium</title>

Or just leave it as a strict product description?

At the product level page, probably just the product description. No need to dilute that <title> if you don't have to. And, if you really need to, put it at the end of the <title>...

<title>Product Description - DomainName.co.uk</title>

Or...

<title>Product Description - DomainName</title>

I'll usually reserve the use of DomainName.com on pages at top level categories. Once you drill down to a product page, the domain name drops off. When you are building page <title>'s based on a breadcrumb trail, having that DomainName at the end on a product level page may be a bit too much. I like short titles.

Use balance in this instance. Move it from back, to middle, to front, and then to back again. Don't always put it at the back or the front. In fact, stay away from putting it at the front unless it is a Branding must. And then, home page and other company pages only.

I only use the domain/company name on the major pages.

main, contact, help.. etc. but not on content or product pages.

I suggest putting the domain name last, i.e. <title>widgets by domain name</title> since it is the key word, not the domain name, that people are searching for. If they already know your domain name, you are just wasting space.

Marshall

As a user, i like to see the domain or store name somewhere in the title for easy boookmarking. Sometimes when i'm shopping for a widget, i like to bookmark the widget-specific pages for a whole bunch of different stores and it helps when the store name appears in the bookmark automatically.

I like to see the domain or store name somewhere in the title for easy boookmarking.

When I'm developing <title>s, part of the process is the "Bookmark Test".

When someone bookmarks one of my pages, I want them to see what the page was about. Those first two or three words are going to be very important. You see, if you think about the user, you also think about the search engine.

But, if you put that company name at the front of each <title> all the time, you may dilute the effectiveness of those. That's why I recommend using company name in <title> on company related pages only.

When it comes to Bookmarking, I've given up on having a company name show immediately, it just isn't worth the trade-off in the SERPs. I'm hoping that if someone is Bookmarking, they know how to rename and organize their bookmarks. If not, their loss, and mine too if you are looking for company name to show on a product page bookmark. :(

The bookmark test is similar to the "tab usability" test. With the increase in tabbed browsing today, I don't want every tab that a visitor may open on my site to show the exact same text (the company name).

recent discussion on related topic
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You have to choose the titles for your pages very carefully, so that they match the content.Having the domain name depends on whether you'd like your site to be found for it as a keyword.However, having duplicate titles on your site pages extremely lowers the ranking ability of them.