Alperus :
I have a budget of £1150 for a complete setup. How much should I spend on the keyboard, mouse, monitor and headphones?
Depends on what you like, mechanical keyboards are the hot thing in gaming, but decent models run to about $70 for decent models up to near $200 for fancier ones with full RGB lighting. Mice, good ones you can find for $20, fancier ones with top sensors and brand names can reach $100. Monitors range from $100-500, if you want higher res and 144hz refresh rates you are looking at $300+. Headphones can be had for $30 for decent ones to $200 and sometimes higher for the best ones.
I'd put aside $300 for a gaming setup for the not actual computer parts. A good starter gaming mouse/keyboard set, not mechanical...