While I sympathize with your situation, as John indicated, there's nothing Apple can do about this. That you hadn't yet paid off the purchase makes no difference any more than you'd be off the hook if your car was stolen or your house burned down while you were still paying for them.
If you set up Find My iPad before the device was stolen, you may be able to log into iCloud and get location information that would be of assistance to the police. Otherwise you'll have to make a claim on your insurance, if you have insurance that would apply. If not, you'll just have to bite the bullet, keep paying for the iPad (Barclay will not let you off the hook), and hope the police get lucky.
These links may be helpful.
How to Track and Report Stolen iPad
Reporting a lost or stolen Apple product
Report Stolen iPad Tips and iPad Theft Prevention
How to recover a lost or stolen iPad
How to Find a Stolen iPad
Apple Product Lost or Stolen
Oops! iForgot My New iPad On the Plane; Now What?
If you don't know your lost/stolen iPad's serial number, use the instructions below. The S/N is also on the iPad's box.
How to Find Your iPad Serial Number