The phenomenon of us here in the States getting more feet per second out of some of our big-bores using alloy bullets as opposed to jacketed bullets of the same weight is due to a lower friction coefficient of the alloy bullet. This is due to several factors, all of which work together to provide a bit of an edge in the velocity department in these guns. Bullet hardness plays a role, as a hard bullet, heat treated as our are (BHN 21), aren't prone to obturation from breach pressure, not nearly so much as those of a lesser hardness (commonly BHN 16 and under for many commercial bullets), also we use gas-check design bullets for these high-end loads, which really enhances the velocity/accuracy potential of an alloy bullet, and almost insures no leading provided the bullet is hard, fits the bore well and has a high quality lubricant. Which is the third leg of this triangle, the bullet lubricant quality we use allows much higher than normal cast bullet velocities, without undue pressure rises and freedom from barrel leading.
Now, about your .444 Marlin prospect in particualar! The jacketed bullets listed above are excellent suggestions, and if you are looking for cast bullets of a little heavier weight to meet your country's velocity minimum thresholds, you can certainly do it, with a variety of the LBT style, gas-checked bullets, the .432"-250 grain LFNGC, .432"-265 grain WFNGC and the .432"-280 grain WFNGC bullets to be sure, and even perhaps the .432"-290g LFNGC bullet as well... still doing some pretty amazing load development with both this and the .432"-300 grain LMNGC DCG bullet as well.
Keep an eye on the What's New box on our home page of this website for our upcoming Part III, .444 Marlin- America's Most Versatile Big-Bore. It will be online hopefully by the first of next week. Will be putting the finishing touches on it and the final load testing this weekend... it should answer many of your questions about higher-velocity loading potential for the .444 Marlin!
It's good to hear from our shooting fraternity from "across the pond" thanks for posting, and we look forward to hearing more from you!