To be fair, _Skittle can build a rig, so you definitely can too. *JK Skittle*

I plead with you, do not waste £1000 on a pre-built, even if it is just too "start off" or whatever, you'll get seriously ripped off. Building a PC honestly isn't hard, take a look at Craig's thread;
https://www.thetechgame.com/Forums/t=7469308/pc-building-tips-advice-thread.html
Also, these videos;

There are also several YouTube video's out there that can walk you through the process of building, which could be handy.

You could buy something like this;
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It's a little over budget, but not much, and you can get your OS from [ Register or Signin to view external links. ]

XFire support, so you can easily add a 2nd 390 at any point. Nice little case with a side window. SSD for your OS and a few games/programs. Nice little cooler. Plus, a 1440p monitor. Cheap kb+mouse combo, I'd recommend saving for a nice mechanical keyboard and maybe a nice mouse too.

You could get a nice keyboard and mouse within budget;
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but you have to get 8GB RAM instead of 16GB, and you get a cheaper, non modular PSU with no XFire support. I feel like a non modular PSU in an 88R would be a pain in the ass, so I'd personally get the first rig, and save for a nice keyboard and mouse.

I was in the same situation as you a few months ago. A long time console user wanting to jump into the PC space. I too was looking around at Pre-builts but for the value of the desktop, monitor, keyboard, and mouse. It did not add up.

Prior to buying everything, I maybe watched 3 hours worth of building videos on YouTube. It really helped.

These two are the best, both from Newegg.
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After getting the knowledge to build the PC, I jumped in and bought the parts, went to PCPartPicker.
Got the best bang for my buck by asking people around the site for help.

If I can do it, anyone can.