Hi everyone,
First of all I'd like to thank everyone who has shared valuable information on this forum, as it helped me make a decision about whether to buy the Sony PRS-505 from the US and import it to the UK.
I decided to give it a go, especially as the exchange rate is very favourable at the moment.

I managed to pick up a previously owned but unused PRS-505 on eBay for $285 + $25 shipping via USPS Priority Mail International. It was shipped two weeks ago so I'm hoping it will arrive any day now. Even though I didn't ask, the seller was kind enough to mark it as a gift which might help with import duties.
Late edit: just heard that it cleared UK customs yesterday morning. Oooh!