I'm assuming that I can (and buy a UK keyboard separate), and that I just need to find a UK power adapter. Is this correct? If so, is eBay the best place, or can I get one direct from Apple somehow?
Yes, Macs have almost always* used auto-switching universal power supplies, so all you need is the right AC (mains) cord or plug. The connector between the AC cord and the power supply is usually a standard connector, so you don't necessarily have to buy a cable from Apple. If you do want to buy a cable from Apple, just go to store.apple.com.
Probably the most significant regional difference today is the Wi-Fi card, which may default to scanning US FCC-approved channels, which may mean it won't find an AP on channel 12 or 13 in 2.4GHz because it doesn't think it should scan them, and it may transmit probe requests on channels (149-165) that it's not supposed to transmit on in ETSI-regulated places like the UK. But as soon as it sees an AP publishing the UK country code, it'll switch into UK mode for that session. No one really cares if a few expats and visitors have equipment that occasionally transmits a frame on the wrong channel, as long as no business is commercially mass-importing devices that break the local rules.
*almost always: Well, some Power Mac G3 and G4 towers from a decade ago might have had manually-switched power supplies. See here.