Firstly, I can't say for sure as I don't even have an Xbox, but I think the registration codes work with any platform as long as you have the game running (the codes are registered to your account, not the game installation, so to speak).

Secondly, you can not buy anything via the PlayOnline Viewer any longer (if that is what you refer to with "or do i buy the expansion packs seperately for £7.99 each from my account page?"
In any case, I would recommend the collection as well since it is cheaper than to buy them singularly like I, and many others had to do in the past just shortly before the cheap'o collections came out and I digress... >.<

Thirdly, the add-ons basically use the earlier, "real", expansion packs (or so I think of it), so they can not be downloaded or installed separately either way. Anything they might need will be downloaded and installed by the PlayOnline Viewer if certain expansion packs are installed as well (Rise of the Zilart and Wings of the Goddess).

Lastly, as I said, I don't own, nor do I ever plan on owning an Xbox, so I can't say for sure that it works, but I think that is how it goes and I also think that I've read someone somewhere confirming it in the past.
Either way, this is just how I think it is so take it for what it is worth, as an experience of mine, and you should not blindly trust it to be the experience you are looking for.


I do hope this helps~
Good luck!