I can highly recommend the wheels built by Strada in West Sussex. They’re happy to build and post and they only build wheels, they aren’t a shop.
I hadn’t heard of some of the rims they carry and recommend before, but having had a set from them I love the way they ride. They are a little more ‘left field’ in that they will take you down the traditional handbuilt wheel route rather than factory built units, but for a Colnago Master, something like that would be a great match.
I have H Plus Son Archetype rim on CK R45 hubs built with Sapim CX Ray spokes.