Did Excitebots ever come out in the UK, and if so where can I buy it?
If not, what do I need to do (jailbreaking/modding/etc...) to get my Wii to play a US copy?
According to Wikipedia, Excitebots only came out in the US.
If you do wish to play it however, I recommend you get a US copy from a store that ships to the UK (I use VideoGamesPlus to get my US games) and soft-mod your Wii to play games from all regions (you can also get a US Wii, but that'll be quite expensive effort for just one game).
Soft-modding your Wii isn't a very straightforward process and can potentially damage your Wii. There are several guides on the internet for doing so, I used the one here but I'm certain there are many other guides online.