I’m looking for a personal recommendation for a Headphones Extension Cable (that is available in UK).
Are you looking specifically for an extension cable, or simply a longer cable?
If its for the Focal Celestee's, then if they are like my Focals then the cables are interchangeable as you are I am sure aware. I wondered about getting a completely different cable for mine as the supplied cable is not very compliant. Maybe @MI' could help with a complete replacement rather than an extension cable?
Another consideration for a completely new cable is that the Prime can also be used with dual left and right channel cables to reduce crosstalk
https://www.Meridian-audio.com/prod...faqs-3whyaretheretwoinchheadphonesockets