Rethus4 :
Do I need a z170 motherboard to overclock my gpu? And any recommendations for a good motherboard under £90
No, you have mainly two diferent overclocks on a Pc:
One is the gpu and they could allways be overclocked (well, not allways it's depend on the card, the bios and the application that the card vendor gives you or if you could use a generic application) because they have no relation with the CPU and chipset you have installed.
Other is the CPU overclock, if you have a K model processor, then you need a Z chipset to overclock it.
Regarding a recomendation you need to specify a bit more:
Do you want/need a lot of USB's?
How many sata devices (Hard drives, sata SSDs, etc..) you have?
Do you want multiple M.2 SSD support?
Do you will SLI/Crosffire?
Do you want USB 3.1C?
You are going to overlock the CPU?
Do you have a discrete sound card, want a good integrated sound card or don't really matter?
Do you plan to play on hardcodre level on fps games and want specific ethernet solutions?
Do you want to pay a bit more for a fancy MB?