Actually, its not moving south, it just hasnt migrated north to allow the continental highs to push in over the UK in the Summer months...!
Its also to do with the shape of the Rossby waves (kind of an 'S' shape in the jetstream, with the long axis of the 'S'
approximately parallel with the equator). If my memory serves me correctly (its a while since I did any climatology/meteorology in my degree, so more knowledgable contributors please elaborate or correct me), its this that causes the jetstream to migrate - the Rossby waves become more acutely S-shaped in the Summer, carrying the jetstream further north above the UK. All related to global atmospheric circulation & just a bit of a blip this year...