It's a simple dominant so to guarantee getting champagne over any base coat you just need a homozygous champagne stallion. He then will pass on 1 champagne gene with every mating.
Ok right Homozygous is rare! Located 2 QH's that are doing shipped semen (no good for UK) one does not take Grade Mares (non QH's) the other is this chap predominatly from working foundation QH lines
Skippin the Bars 4866311 2006 http://www.skippinthebars.com/breeding.html
It might be worth an approach but to export frozen from the USA the horse must undergo health tests and quarantine so it may not be feasable.
There is 1 heterozygous stallion in Czechoslovakia who is an American Paint and does chilled semen which should be fine to the UK (Ask AP before you consider this) http://www.tworiverranch.com/engpaarden.html