It's stuff to put in a computer, we highly recommend you build your own. You can't get a case for nothing, like ez said, get a good well ventilated one for ~£60-80, very much within your budget.
Let me suggest this quick list from OCUK, are you wanting to include a monitor/mouse/keyboard? Or do you already have these. If so, what size monitor as it can determine the graphics card you get.
Obviously if you need an operating system, monitor, keyboard etc added in I can change it to incorporate them for under £800. Also, don't be put off by having to build it yourself, it really is a great way to get to know how these things work, and it's very useful to be able to take a PC apart and know what everything does. There are also loads of guides to help you do it.
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