So here's one of the benefits of the Android being open-source - there's an application available which does the Android equivalent of jailbreaking (called "rooting") for you. It takes most of the "heart-in-mouth" bits of the process out, so that anyone can do it.
The benefits? You can do custom OS upgrades (like installing HTC's Sense UI on older Android models), tether the handset to your PC to act as a modem, or even back up your operating system onto an SD card.
The App is called "Recovery Flasher", and it's been produced by someone called RyeBrye. All you need to do is download the app, sideload it to your G1, run it, tap "back up a recovery image" and then tap "Flash Cyanofen Recovery 1.4". Easy.
From there, you can install some of the features that are in forthcoming Android updates, like multitouch, give yourself new homescreens or system keyboards. There's also the option of "vastly" improved performance on the handset.
The software says that only US Android handsets are supported, but UK ones can't be that different, right? Right? Actually, we're not sure about that one, and doing all this is at your own risk. So be careful, okay?
Update: We've had confirmation from at least one UK user that RyeBrye's trick worked on a UK HTC Magic. Anyone else had any luck?