First, (sorry to doubt you...) are you SURE you have a "computer" voice selected? Originally the the version of the XL released in the UK was not able to use computer voices at all, but the re-released version you have (the "classic") may... I'm not sure.
AFAIK, the only English computer voices ever used on the XL are "Kate" and "Simon"
Next, once you have selected an appropriate computer voice, have you ensured that "read aloud street names" is ticked in the "Voice Preferences" screen?
On my XL Live, you can only get to that screen by re-selecting a voice on the "Change voice" page first. Once you see the silhouette of the person and tap "Select", you get to the hidden preferences screen.
As I say, I have an XL LIVE, so my menus may be rather different from yours, but if you go to Main menu / Services / Speed cameras do you have a button labelled "Enable alerts"? (If it says "Disable alerts" then they are already activated).
Having that enabled, should light up the other icons, including one called "Change warning preferences".
If you tap that, you should get a whole list of camera types and each one in the list should have a small blue loudspeaker icon to the left of the name.
If they don't, then you need to do the set-up for each camera type again.
Even if they already have that loudspeaker icon, tap each one in turn and check that the option on the next screen "Warn me when approaching a camera" is set to "Always" and not "Only when speeding". Otherwise you'll only get alerts if you are already speeding!
Finally, I should mention that the TomTom-supplied camera data is actually not very good. There are several third-party camera databases available which are significantly better. My preference in the UK is to use the PocketGPSWorld camera alerts, which are MUCH more accurate and complete than TomTom's efforts.