Hello @Scouse_42 

Sorry for the late reply. 

Thank you for being interested in tp-link OneMesh feature. Here is basic introduction of TP-Link OneMesh 
https://www.tp-link.com/onemesh/

OneMesh is built between router and Powerline kit or range extender, and users will need a tp-link OneMesh compatible router to create a Mesh network with OneMesh compatible powerline units.  From here you could find the tp-link OneMesh compatible router more QAs about OneMesh: 

Several questions about the OneMesh™ and Mesh

If the ISP has 1 Gigabit speed, you could consider an OneMesh router with Gigabit port and select one regarding the Wifi spec 
For powerline KIT, you could consider AV1300 Wi-Fi powerline kit, like TL-WPA8631P KIT V3, but they usually sold in KIT with one wired and one wifi unit. 
If you could run Ethernet cable between router and different extenders, you could also consider Deco Mesh system. 

Deco Ethernet Backhaul feature and Topology

Kindly note that the Deco system is different from TP-Link OneMesh >>> TP-Link OneMesh™ VS Deco Mesh: What's the Difference?

Hope this could help~