As above, if you aren't already using an SSD then that would be a great upgrade.
Upgrading from 8GB ram to 16GB isn't really worth it unless you have a specific need (as 8GB sticks are disproportionately more expensive, and DDR3 can't be carried forward to a new build)
Upgrading the processor probably isn't worth it - the i7's that fit this socket again cost a premium (£100+) which would be better spend on a newer motherboard and CPU.
Upgrading the Graphics card (GPU) will make a big difference though for gaming - although to save money you would be better looking 2nd hand.
A used Radeon RX480, RX570 or RX580, can often be had for around £70-£80, or Nvidia Geforce 970/980/1060 can be had for £70-£130, but prices on Nvidia cards may drop further as a new generation has just been released and people upgrade.