If I’m reading correctly you just want a 3m or 5m with rj45 on both ends ?
Europe wide there are plenty of options , and in England there are a few options from amazon cat7 or cat8 patch cables , to official b&o , to sounds heavenly , to ebay sellers , to making your own or getting a network company to make them .
If it’s old powerlink to rj45 , I’d only go with sounds heavenly .