Someone else has mentioned Jephyrr, he's perhaps the most active, even still during Unity and has been around since the first few months of the hotel. There's screenshots of him from late 2001 hanging around public rooms, the dude's a legit player who might not've been active all throughout in the interim period but is sure back with a vengeance now, with a whole host of huge new room designs to boot.
See screenshots of Habbo UK, 2007 onwards: Habbo Archive
Got your own screenshots from pre-merge Habbo UK? Contact me
All furniture released, ever, in one place: Museum of Habbo
Have a piece of furniture the Museum needs? Contact me