Let's be clear about this, the donation money isn't just used to keep the encyclopedia online. They have hundreds of language versions of Wikipedia itself, and many related "sister" projects, with perhaps the most clearly impacting being Wikimedia Commons (a huge repository of free-content media with a focus on educational use - including much media in the public domain) and Wikidata (a project that has now emerged a…
And perhaps more to the point, Wikipedia has legal personality in the UK, so one assumes the above applies.
But really it would be pointless: I'm not going to tell Wikipedia what their strategy should be when I donate. So even if you do, they can put my donation to whatever purpose it is that yours precluded.