Not only fish consumption was pretty much abandoned in Britain and Ireland with the arrival of Neolithic only recovered under Viking influence many millennia later but the most striking fact is that it was replaced by milk as main source of proteins. This fact, considering that farmers studied in Central Europe and Iberia have systematically tested negative for lactase persistence, really opens an avenue for the possible origins of this nutritional adaptation because it is most unlikely that they were such notable dairy consumers without the corresponding digestive ability even cheese may be harmful to lactose intolerant people unless it is aged, while yogurt was almost certainly not known yet in Europe. While the evidence comes from the Atlantic Islands, it is worth to notice that their chronologically late Neolithic has its origins in the much older agricultural cultures of NW France, another blank spot in the ancient DNA map of Europe. Nowadays NW France is high but not particularly high in this phenotype but SW France and Basques have among the highest LP scores both phenotype and rs4988235T genotype in Europe, together with the Atlantic Islands and Scandinavia. Then again it is worth recalling that one of the first areas where the rs4988235T allele is found is in the southern areas of the Basque Country, with clear signs of two different populations one lactose tolerant and the other lactose intolerant being still in the first stages of contact and mostly unmixed.This leads us to the issue of Atlantic Megalithism tightly associated to Atlantic Neolithic and its still unsolved, but likely important, role in the conformation of the modern populations of Europe. Whatever the case the first farmers of the islands were heavy dairy consumers, although in Britain but not in Ireland and Man they eventually derived into heavy meat eaters later on:

via For what they were… we are: Neolithic peoples from Britain and Ireland ate a lot of dairies and nearly no fish.