Not sure if talking about a prebuilt PC or actually building one. I would suggest building your own PC for more than on reason, in short, it gives you full control of what you can do with the system down the road and gives you experience of building your own system, while also saving you money.

You're going to have to give us a budget and a currency. You can't say "that doesn't cost crazy amounts of money" because "crazy amounts of money" could be different per person; $400 might be a lot to one person where as $2400 might be a budget for another person and going over is a lot to them. We also don't know your financial situation, we can't do a build that is going to break your bank, only you know how much you can spend.

A little more information is also needed.

Do you need any peripherals such as mouse, keyboard, monitor, ect?
If you need peripherals; are they included in your budget or their own separate budget?
If you have a monitor on hand, what is it's screen resolution?
Do you need an OS?
Is this going to be used for anything other than gaming? Such as work/school, video/photo editing, music production, ect.
Any aesthetic choices you care about such as colour scheme, tempered glass, ect?

Would say around £800

Just want a fast reliable pc that runs good while playing games.

Games like
- h1z1
-battlegrounds

I don't know how to build a pc, don't know what's a good gaming pc

Would say around £800

Just want a fast reliable pc that runs good while playing games.

Games like
- h1z1
-battlegrounds

I don't know how to build a pc, don't know what's a good gaming pc

You're still not giving us enough information.

Will you use the system for anything other than gaming? If so, what?
Do you need a monitor? If so, do you need it included within the £800 budget?
If you already have some, what monitor(s) will you be using with the system?
Do you need any other peripherals?
Will you overclock?
Any preferred aesthetic(colour scheme, side panel window, LEDs, etc)?