What are the downsides of getting a foreign keyboard layout on a MacBook Pro? A 13-inch MacBook Pro costs £999 in the UK, compared to $1,199 (about £760) in the US. 

For this reason (I guess), brand new US models cost about £850 on UK eBay. The obvious downside is the keyboard layout is US, not UK.

Only a couple of keys are different on US keyboards. So if it's possible to change system preferences to interpret it as a British-layout keyboard, then I'm happy to put up with a couple of wrongly labelled keys, if it saves me £150.

But are there any other downsides I should bare in mind? Any possible incompatibilities etc?
I once was in a similar situation to what you described—I'm an American and brought my U.S. MacBook to the UK.  A couple observations:


U.S. keyboard layouts are substantially different from the UK ones. Many common punctuation characters, including quote marks, are mapped very differently. I found UK keyboards too awkward to use and did virtually all of my work on my own U.S. keyboards.
U.S. keyboards do not include the pound sterling symbol or the negation symbol used in the UK. You can get them by using Option key combinations, but that can be a bit of a pain.
As bmike mentioned, if you need a keyboard repair you'll face a substantial delay. Someone spilled a drink on my MacBook in a pub towards the end of my stay, and about a third of the keyboard was shorted out; when I took it in to an Apple Store, they warned me they would need to hold the computer for several days while replacement parts arrived.  I couldn't spare the laptop for that length of time, so I spent the last couple weeks of that trip using an external keyboard with my MacBook, and repaired it when I got home.  (That option, of course, won't be available to you.)


All in all, I don't think I would recommend it for you.  That £150 is buying you freedom from a lot of hassles.