If Hendersons', then from wiki
Availability
It can be found in local fish and chip shops and almost every supermarket and grocers in the city, yet is almost unavailable anywhere outside South Yorkshire (although it can now be ordered online in multiples of a litre).
The company has made little effort to market outside Yorkshire, instead preferring to encourage a loyal following nationally through this sense of exclusivity. Although often available throughout the UK, this has been due to the efforts of individual shops, and loyal Yorkshiremen smugglers, to obtain small quantities for their local clientele.