There are lots of places in the UK to shop, start with
www.dabs.com but keep an eye at
www.todayonly.co.uk for dailiy bargains on bundles etc.
I would strongly suggest a Duron 1G or 1.1G (the 1.2G is improportionately expensive) and a board based on the SIS735 like the ECS. I always recommend a 7200rpm drive even for office use, because it has the largest impact in overall performace. You'd better drop 200Mhz off the CPU speed and stick to a 7200 rpm drive. The Maxtor 740 is my favorite.
For graphics card get what suit your needs, if you're not gaming any card up to £55 will do more than fine!!
256MB ram is the absolute minimum for these days, and DDR is definitely the way to go.
If you are not into gaming, I would suggest a nice all in one SIS740 based board like the PCCHIPS 830LMR. It is dirt cheap and comes with modem/NIC/Sound/GFX. Some don't like the idea but trust me I got the same setup here and works like a dream. yes the softmodem + hardware DVD support etc. For £60 or thereabout you get this board and you save on all the hardware it comes with.
Like I always say, go by your needs and don't get carried away.
If you're after a burner, skip the current 24x madness and stick to a £60 16x LG.
You can PM me if you want more info for shopping in the UK