Well I am also from the UK, as you know Yoshi

but I signed up for the Beta testing and have been playing for a while. I have Unreal Championship, Ghost Recon and MotoGP (bought the full game after playing the LIVE demo). The game that works best on LIVE has to be MotoGP. It is almost lag free, even against American players, has great graphics, great gameplay and I love the lobby feature after races where you can all talk, discuss the race and have a laugh. It is a really social experience. The game that works worst is Ghost Recon - very laggy, hard to find stable games etc. However, when it works it is another fantastic experience. A nice slow, strategic game with tons of variety in game modes. Voice comm adds a whole new dimension to this type of game and really helps both in gameplay and in atmosphere. Unreal Championship is " greatish" (a Pierreism). I love UT2003, but as Yoshi knows, I have struggled to get into UC. Nice level design, nice graphics, crap framerate and unbalanced character classes sums it up for me. Championship mode solves the character class issue though and the patch is supposed to help the framerate so there is hope yet. Most of my problems with this game stem from the fact that I am too used to playing Ut2003 and find the control and slight differences restrictive and annoying now. Regardless of my issues with the game I would strongly recommend it to anyone who buys Xbox LIVE.
Overall I would rate MotoGP my favourite just now, but I am really looking forward to Capcom vs SNK2 and Halo 2.