Tig, I don't know if there would be any prohibitions against importing pussy willows from the US to the UK, but I have a feeling it would be cheaper to order them from a commercial nursery than to pay the shipping from Oregon to the UK (complete with the type of packing necessary to keep them alive en route).

Within the US a small package can usually make it cross country in 2 or 3 days by normal mail or UPS, and small cuttings would not weigh much.

Another Q for you botanical gurus:   When taking a cutting from a pussy willow to start a new plant, how important is it to keep the cutting (the base) wet/moist until it's stuck into the ground?
E.g., the branches I cut earlier this week are on my porch and NOT in water, because I don't want the catkins to bloom.   These are intended mostly for arts and crafts use, but I'm curious as to whether they would still be usable for starting new plants after they've been sort of dryig out for a week or so.