Considering Nook? Other options available.
 

I'm thinking of buying and ebook so I can read PDF documents (mainly) such documents will usually be technical books (java, c++, c# etc. etc.).

The key things for me are 6" screen, expansion, hackable/customisable OS/firmware (i.e. linux or CE), wifi, fast processor. To this end the Nook seems to be the best and cheapish.

Are there any other ones that meet this criteria that I should consider?

Wireless plans, DRM are of no consequence (hackable OS negates DRM).

Oh and need to be able to buy them in the UK - somehow.

You're going to be disappointed with any 6" device if reading technical PDFs is your primary aim; no 6" device can do so satisfactorily. I'd suggest that you consider something like the 8" iRex DR800S instead.

Yes but thats twice the price, I'm not looking to pay more than £150ish.

I'm afraid you're out of luck. There is no £150 eInk device that will display technical PDFs well.

You may have got your prices for the Nook a little wrong. It costs US$259, which is about £170. Add to that about £20 for shipping, and 17.5% VAT on the whole thing, and you're looking at around £230 to get it here in the UK.

Yeah I thought as much - anyway the Nook isn't available in the UK, so I've looked at the sony PRS600, but my question is that can custom software be added so that I can slot a wireless SD card and be able to use wifi?

I agree with HarryT that no 6" EInk device is a good choice for PDFs. However, some are better than others, and two with relatively good support that meet your other criteria are the PocketBook 302 and the BeBook Neo. The PocketBook's resistive touch screen (like the PRS-600) is probably inferior to the Neo's Wacom touch screen, but the PocketBook is more hackable and has the best available PDF support on a 6" EInk device.

Both of these are better than the PRS-600 for PDFs, because the Sony does not have persistent zoom.