Mine are working fine with the powerline adapters. I'm sure they say somewhere using powerlines may work but isn't officially supported as peoples home wiring varies and impacts on the effectiveness of using the adapters.
As a previous poster suggest if you are not happy with the Ethernet cable then your only option is to buy a pair of 500Mbps HomePlugs and if they do not work then simply return as faulty to the retailer.
Here is a cheap suggestion -
TP-LINK TL-PA411KIT AV500 500 Mbps Powerline Adapter Starter Kit - £24.99
BT Infinity (EASTF) 58.93/15.94 Mbps [IP Profile: 60.8/20 Mbps] using Openreach Modem & Billion BiPAC 7800N