Keep an eye on large supermarkets closer to you then, issey. My experience over many years is that these offers can (not always) mean that the manufacturer is doing a discounted offer to the big supermarket chains - so it's possible you might see it in others as well.
Pets at Home often have the same offer. They don't have it at the moment though. And I just did a Tesco online shop and they only have the 6 fish pack there - not a 12 pack and not the meat one - very disappointing as they normally have it in store