Now confirmed by Amazon that this doesn't apply to the UK, but the limit will apply when you travel outside the UK - so same as the US then.
"Beginning on July 1st, 2012, web browsing on the Kindle Keyboard experimental web browser using a 3G connection will be limited to 50 MB a month outside of the UK. You will still have unlimited access to Wikipedia, Amazon, the Kindle store, and the ability to download content" and "I can confirm that there will not be any limit in the use of Kindle keyboard over 3G in UK for UK residents."
So, looks like HarryT was spot on with his thought that it's capped in countries where Amazon doesn't have a roaming arrangement with a local telecoms operator. I don't know which countries other than the US and the UK has agreements, anyone else?