Can anyone direct me to a seller preferably in the uk with 8mm thick show leads or sets, neck piece or collar must not be 'slip' but must have a 'stop' or slide to endure it can't tighten in dogs neck if pulled if you know what I mean.

That is very thick, I do 2.5mm braided and 4mm paracord [550lb breaking point] but don't know of anyone doing 8mm.

Sorry but the style you want means nothing from your description.

i presume you mean like the bottom one in this link a gundog lead http://www.workingdogcompany.co.uk/results.php?category=3 sorry dont know of any suppliers for equivalent show leads though