You can always take a class of some sort. I took a class all about computers last summer, and I have the knowledge to build/or customize my own.

If not,

Heres 3 book -
http://www.amazon.com/dp/0072255595?...79A9KY4PFPPCD&
http://www.amazon.com/Building-PC-Du...pd_sim_b_img_6
http://www.amazon.com/Build-Ultimate...pd_sim_b_img_6

I live in America, so I wouldn't know any stores in the U.K.

If your internet cafe is for people who just surf the net, luckily, its gonna be pretty cheap.