Hi,
The maincrop potatoes sold in the shops are treated with a growth retardant to stop (or at least slow) them sprouting.
I forget the name of the chemical used but it is only allowed in minute quantities and there have been concerns in the farming press that it may be withdrawn as many batches have been testing over the limits.
Klassmann compost can be had from BHGS/JFC Monro I think, if you are anywhere near their branches.
Regards
Tristan