You can already re-download apps and books that were purchased in Apple's App Store and iBooks Store. If you are in the U.S. then the iCloud beta is already running some services, including the ability to re-download all your iTunes purchased music.
I don't know if the last is showing up in the UK yet, but if you go to the various stores in iTunes on the computer, or the iBooks Store, App Store, and iTunes on the iPad there may be a Purchased icon at the bottom of the screen. Tap that and you will be able to see everything that Apple says is yours, free to download again.
The Apps and Books can be downloaded again even is this button does not show, but you are on your own remembering and searching for them. they should show with either a Installed or Download button in place of the usual price tag. You can also email them to your self, though this could be tedious if you have a lot of photos.
For your music, don't' go buying it again. When iOS 5 and iCloud come in the fall (hopefully to the UK at the same time as the US) you should have access to all your purchased iTunes music.
For content that is still on your iPad, you can right click on the iPad in iTunes and choose Transfer Purchases. This will move all your Apple Purchased content back to the computer. It will do nothing (probably) for content you purchased or added from elsewhere, but you'll find that most of this can also be copied from the iPad either in the app (using email) or with third party programs on the computer (sorry, don't remember any of the names).
You photos, however, are gone, unless you have backups on the computer or copies of them elsewhere. iTunes only syncs photos one way, to the iPad. When you re-synced with iTunes and the photos were not on the computer iTunes removes them. This is the normal way to remove photos (synced with iTunes) from the iPad. Photos was only meant to be a way to view a few albums on the iPad, so it's not designed to be a good backup device. At least not yet.
The photos synced with iTunes are not even full sized originals, so recovering them can be a disappointment even when it is possible.
Sorry.
P.S. I reread your post, and it appears that some of your photos may still be on the iPad. In that case you can use a third party app like PhotoSync to copy them to the computer. Other than the camera roll, saved pictures, and imported albums the photos will probably be less than original quality, but better than nothing if that's all you can save.
