Goodbye Vodafone UK Nexus One ROM! Hello Gingerbread. The Open Source Administrator
This link looks like it contains all the information you'll need to do this yourself (as well as some details about why you're stuck at 2.2).
The basics are that it drops the phone all the way back to bone stock, then rebuilds Android up from there, but using the US Google (unbranded) release and updates. This should clear out the Vodafone ROM. Apparently Vodafone is what's stopping you from upgrading since they haven't released 2.3.
Warning, I haven't gone through these instructions, and there is always a chance something will get messed up. So you should make sure that you're comfortable with these, and maybe verify if others have had success.
I read through more of this, and it looks like all you really have to do is downgrade to the base Google release of Android and the OTA updates will start to come in. So you don't have to do the rest of the manual updates if you don't mind waiting. Apparently they start to come in pretty quick.