"This adaptor works for the money. Not sure about CAT6 cable, mind-you, as I don't use it.
I tried with another cheaper coupler and only got 100Mbps.
Adding this adaptor and an old CAT5E cable as the extension, allows connection at gigabit speeds. Although, throughput has so far been limited to maximum 600Mbps / 75MBps (not fully troubleshoot-tested as to where the bottleneck is, my HDDs can go faster - but it's promising).
I had to re-site my personal setup and for some reason, the cable didn't have any spare length in it. This is why when running a simple cable, to a non-fixed, non-wall-socket setup, in a standard-sized room, always add enough spare cable to run around the wall to ANYWHERE in the room, then add a couple of metres to that - and keep the slack bunched-up somewhere behind the desk).
Screwfix (not the source of my cheaper coupler or the extension cable but the source of my CAT5E main cable run) sell cheaper CAT5 / CAT5E couplers, but you're already introducing some impedance to the signal by splitting the cable, so for a couple of quid extra, you may as well seek the best performance."