Postage is £30 on top, no operating system, cost of components is:
Motherboard
http://www.asrock.com/product/product_k7s41.htm about £32
DVD Writer about£40
Memory about £28
Hard drive about £37
Floppy and modem about £10
Case+PSU about £25
Processor+fan about £98
Total cost of components = approx £270
Probably a bit cheaper as that was using commonly available component prices.
Worth having a look on the Dell site as occasionally they have some real bargains. Also, since you are a teacher IIRC, you may be able to get an educational discount - try asking your IT people.
Typical PC base unit I put together for people these days comes to about £400ish for the parts (3.2Gig Athlon 64, 160G hard drive, DVD writer, low end/embedded graphics card, 512 DDR, motherboard,cheap case, keyboard, mouse, floppy). With a 'reasonable/decent' card e.g. the 9800XT you have to add about £125
Plus Operating system of course
(That wasn't an offer to build just an example of what you could get)