I have just looked at the kippers on the Tesco site.
Only one type is shown on the fresh fish stall -
"Counter Scottish
Dyed Smoked Kipper Fillet"
The ingredient list doesn't specify the dye used.
It is better to eat Manx kippers. They may well be thinner but the 'thinness' is due to the
natural curing process.
http://www.booths.co.uk/producer/manx-kippers/