Do you have the LIVE model? If so, the alerts are enabled in Main menu / Live Services / Speed cameras/Enable alerts.
Are they enabled and still not working, or can't you enable the alerts in the menu?
If you have a backup, you can easily restore the cameras either manually, by copying the files from the map folder of the backup to the map folder on the device.
If you have the "Premium camera service, the UK files are called "Safety_Cam_UK_Premium.ov2", "Safety_Cam_UK_Premium.ov2.dct", and "Safety_Cam_UK_PremiumOv2.ver
The European cameras all have similar names for the other countries, or may be other .ov2 files such as "Blitzer" or "Fartkamera". (Always makes me laugh that one....)
You can probably also do it by using Home and "Manage my device/ Items on computer", but I never bother with Home for things like that.