Quick question: how comes the configuration? Fully decided yet, if I may ask?
Also, as a 14 old boy (not doubting his skills) can fully put together and install a desktop computer?
EDIT: I would slightly modify the previously suggested build.
http://uk.pcpartpicker.com/p/bKRsjX < £633
Proposed case with side window for £57
The newly proposed build got a non-BOX quiet CPU cooler, bigger SSD (256GB vs 240GB), bigger HDD (2TB vs 1TB), slightly faster GPU with efficient and quiet cooling, while still leaving enough from the £700 budget for a case of your choice with desired & respectable qualities.
Review for MSI R9 280.