We've got Nokia N8 unlock codes
available but none for Tesco Mobile.
Basically, there is no source for all Nokia handsets, like there is for say HTC. And there is no source for all Tesco Mobile handsets (or O2/Three/Virgin). Which means if you have any Nokia handset on Tesco Mobile then your only viable option at the moment is to call Tesco Mobile customer services and see if they will issue you the unlock code.