OK.
In which case, I'd avoid an all-in-one as they tend to be like laptops and are more difficult to fix. If it were me, at that budget I'd go for something like:
http://www.ebuyer.com/411064-zoostorm-desktop-pc-7873-0446
or
http://www.ebuyer.com/395297-zoostorm-desktop-pc-7873-0436
depending on whether you want windows 7 or 8. Both of those have a nice fast processor, decent memory and a big disk. Not massively expandable, but standard PC parts that you can swap if you need to. Zoostorm are e-buyers own brand of PC (I buy a lot of stuff from e-buyer so don't be worried if you've not heard of them before) and are aimed at businesses. They are towards the no frills end of the market, but you do get a lot of PC for the money (these will be quite a bit faster than the all-in-ones - even at £600). Something similar from one of the known "build me a PC" specialists (Novatech, Chillblast) is going to cost another £70 at least. See:
http://www.novatech.co.uk/pc/range/novatechlifenti29.html the £400 price is WITHOUT AN OPERATING SYSTEM)
Then buy a decent monitor (you WILL notice the difference between these and a cheap monitor for photography). Something like:
http://www.ebay.co.uk/itm/Dell-24-inch-computer-monitor-U2412M-IPS-Silver-1-year-warranty-/200884495734#vi-content at £179
(there are a number of people who have bought from nrg_it on here and been happy). If you want the same thing but new, it is available here:
http://www.cclonline.com/product/65168/U2412M/Monitors/Dell-UltraSharp-U2412M-24-Widescreen-LED-Monitor/MON0494/
for £215
You will be surprised at how much quicker these are than a 6+ year old Mac....