Not sure that the extended warranty would have covered accidental damage anyway?
Your only option is to scour Ebay for the part and hope the laptop is not damaged elsewhere. I'm guessing that a motherboard would cost than £30-90 second hand. Fitting it should be fairly easy, and a local PC repair guy would do that for £40 or less.
There's about 20 listed on Ebay now, and you could set up a search that will alert you if your "model number AND motherboard" was listed.