If you expect to buy new releases in the UK, then a device that supports Adobe ePub is best. MOBI is more popular in the US, but in Europe ePub is king.
The Sony PRS-505 is a safe choice. It was the first EInk device to support Adobe Digital Editions (ePub and PDF).
A brand new option is the
COOL-ER Reader. It has not been released yet, but early indications are that this will be an unexciting device. It does support Adobe ePub though.
Other Readers will soon support ePub, but don't do so yet. I like the BeBook line, which will soon include a 5" device and a high-end device. Currently the original BeBook supports MOBI, but there will be firmware that supports Adobe ePub (we know this because the EZ Reader, another Hanlin V3 brand, in the US will very soon have ePub and so far all firmwares for the Hanlin V3 work on all devices). If I was buying a BeBook in the UK, I would make sure it ran Adobe ePub before buying. I don't think this has been confirmed yet. This is why I say that the PRS-505 is the safest choice.