First of all, congratulations on completing your site.
My first impression of the home page is that it's a little confusing. I would like to see a paragraph or 2 explaining the purpose of the site and maybe some kind of introductory text.... "Welcome to PenguinPages. Help us build the most comprehensive UK business directory on the web. ..."
I really like the mascot. He looks fun, fresh and a little bit silly. Well done.
I would like to see some of the content moved a bit toward the right of the page. The composition looks off center to the left just a bit too much. Even if you moved the mascot over a bit it would help.
I like the colors and the logo as well.
Your spelling looks pretty good. I didn't see any errors.
I would like to have the ability to browse the listings by category. The purpose of a directory should be to compile a list of sites that the average person can use. People then have a place to go and look for businesses that can help them fill some kind of need. If I, as an average person, need to know the name of the business (Who) or the address (Where) then that assumes that I have knowledge that I may not possess. If I already know the name of the company and the location, why do I need a directory? As far as the "What" search bar... lets say I need someone to lay bricks, so I type in "brick layers". What if I would be better served by typing in "Construction" or "Stone Masons". Having the ability to browse by category would let me easily navigate to the correct listing.
I think too many of the people that create directory sites have the wrong idea about them. Many think that a directory is an SEO tool. They look at the directory simply as a place for businesses to submit their sites to. That is like saying that a telephone book is a place to sell advertising and collect phone numbers. This may be true, but it's not the whole picture. Web directories, like phone books, are essentially tools for the average person to use to get information.
Also, by letting the viewer browse by category, it keeps the viewer on your site, going deeper into the directory pages. This might lower your bounce rate and give you many more page views.
Your search page seems to be just another version of the home page, it's just set up a little differently. This means that one of them is either not necessary, or not set up correctly.
You have that "Custom Search" box on every page. Since you have a dedicated search page, is it really necessary?
Give the site some content. Why not have "Featured Listings" on the home page. Something that can be updated.
Overall, I like the site. It's clean and simple. Try to eliminate rendundant features, as they can make the site boring and unremarkable.
What if you crated a couple more pictures of your mascot in different poses, maybe interacting with things on the site.. holding the "submit" button or pointing to the featured site. I think you could have some fun with it.
I think you're off to a great start. Have fun with it and remember to make those T-Shirts.
Good Luck!