It's not just humans, other mammals, who have not been affected by Neolithic or the Kurgan invasions as such, also show that pattern of genetic specificity at the edges of Great Britain. The animals (voles and shrews) show two unrelated clades, one of them more closely related to the continent, which arrived in a second phase, pushing the pioneering clades into forming a "Celtic fringe" of sorts at the periphery of the island.
Source: Jeremy B. Searle et al. The Celtic fringe of Britain: insights from small mammal phylogeography. PNAS 2009 (freely available, early publication).