Personally from experience I would suggest going for an older system along the 2nd hand route. Kids have a nack for tinkering and breaking things and at the end of the day a £100 desktop being fixed up is usually cheaper than a £300 new machine.
This is especially so when it comes to software support that is usually not covered in a warranty. I would trawl the likes of gumtree.co.uk or even freecycle and just look for a bargain. An 8 year old is not going to need the latest hardware. Kids games for that age range dont tend to be very taxing for a computer and for the likes of student software the money you save will allow you to get a few bits to start them off.
Being very honest here, PCA has in the past offered encyclopaedias on the front cover, usually the last years edition but for school/college use it isn't going to really be that much of an issue.
And as for a need for an office suite for an 8 year old will do with just open office as it is free and fully fledged as Microsoft's iteration of an office suite.
Hope that helps with a few further ideas.