How to add Wikipedia as a search provider for the Safari search bar

I don't use it, but GLIMS looks like it will easily let you insert a search engine different than what Safari bundles.

More intrusive, but way slick in my opinion is the SafariOmnibar it's excellent work and you can customize your search provider since it's hacked into that part of safari using SIMBL - the SIMple Bundle Loader process.

I use LaunchBar and search from a keyboard shortcut - it allows extensive control over searching - you can define custom search sites and criteria easily. It has many other features (notable extremely quick app and file launching as well as a clipboard history) but it's so fast you might like it just for getting a UK search if nothing else fits your needs.

The AnySearch extension allows you to set any search engine you want.

It achieves it by listening to the beforeSearch event, canceling the default behaviour and checking your settings for your chosen search engine. Then it puts your query into the specified search engine and displays the results.

You could, too, edit the host file, redirecting form any of the default search engines to the one you want, but that wont be so neat.