Hi man, I tried to make a PC for you with the prices from pcpartpicker.co.uk. I did not include an hard drive. I've gone for a 240gb SSD. when that thing is full, you can always go for an extra hard disk.
now you wanted a green and black pc. I found a green / black motherboard. it's the Gigabyte g1.sniper z87. It might be a little too much for your built, but it's black and green.
honestly, when i built my first pc a year ago, I went for all these colors and stuff. but I'm not happy with it. I was happy with my computer for the first few weeks, but now I just want it to be hidden somewhere next to my desk. I went for the NZXT Phantom 410 white which is quite a noticable case. I wish I'd gone for a more subtle case. now if you would go for a buildt that was less noticeable in your room, you can go for another motherboard that is a lot cheaper and smaller. take for example the MSI Z87M-G43 it has all that you need and is a lot cheaper. if you would put that in a closed micro-atx case, would be great!
Now you wanted a black/green built so i put that in the list below with the prices. tell me what you think!
Cpu - I would go for the i5-4670K it's great cou (I have it myself). you can overclock also overclock this one because it has the K at the end which means that it is unlocked.
Motherboard - I went for the Gigabyte G1.Sniper. again, it's a little overkill but it is black and green (and more expensive).
Ram - Kingston Beast 8gb. It's black, it's cheap. It's what you want.
GPU - I went for the MSI N770 Twin Frozr. It would be one of the more expensive components of your built. now I haven't really followed the graphics cards last year, so you should look this up for yourself a little more. try comparing it to the AMD GPU's.
SSD/HDD - I went for a Samsung ssd. I've got an SSD in my computer, and I could not live without it. I highly recommend saving uo for an hard disk later on and live with this ssd for the first few months. and really an hdd is not that expensive. But I did leave it out.
PSU - PSU's are boring purchases. but it is important that you take the right one. When I built by computer i did not know which one to take, so I went to the store and asked. I took thw Corsair CX 600M. I think you should take the same one. 600 watts is enough for this built and the Corsairs are reliable.
monitor - Benq
case - Corsiar 450D. It's black. It has a window so you can go and look at you green. It's what you want. I suppose there are other cases out there that are cheaper. I haven't looked into this very much.
There you go. that's 929.58 pounds!