Many people seem to recommend
EasyCAP ezcap [apparently the name changed...not sure if this is true or if this particular device is "fake"...] devices (such as :
http://www.amazon.co...=citisupport-21 - about £20 including postage). I personally have no experience with them, but they are definitely an option.
Another possibility is a DVD recorder you may already own. If you have a DVD recorder for use with your TV, you may be able to plug the Wii into one of it's inputs and record that input. For example on my DVD recorder, there is a flap on the front behind which is a composite connection (red/white/yellow leads) into which I could plug a console, and then I record "AV3" or something like that. Then I just can just rip the DVD with my computer. I think mine also has an HDMI input as well (or it may just be output...); if you have a DVD recorder, take a look at the available inputs and if you can find one that fits, turn on the TV and change the input on the DVD recorder until you see the input from your console, then record that input.
EDIT: Found some more devices, under the "EasyCAP" name (really not sure if the company actually changed their name to ezcap or not):
http://www.amazon.co.uk/EasyCAP-Video-Audio-Capture-Card/dp/B0011N9QNC - £4.46 (dirt cheap, probably fake and poor quality)
http://www.play.com/Electronics/Electronics/4-/13703830/EasyCap-USB-2-0-Video-Audio-Capture-Device/Product.html - £14.99 (one of the reviews says it could be fake, but it still seems to get good reviews)
And another with the "ezcap" name:
http://www.amazon.co.uk/Ezcap-Capture-847-Transfer-interface/dp/B0030U57ZA/ref=sr_1_9?s=electronics&ie=UTF8&qid=1322253708&sr=1-9 - £14.99
I think the "ezcap" ones are real and the "EasyCAP" ones are probably fake (there seem to be hundreds of clones, which is why the company changed their name).