(Reuters) -Shell Plc said on Monday it had made a final investment decision to develop the Jackdaw gas field in the British North Sea, which is expected to come online in the mid-2020s and could provide 6% of UK gas output. The Jackdaw project, which had been initially rejected by British authorities, received regulatory approval earlier this year after Shell changed its environmental development plan. The decision to definitely move ahead with Jackdaw comes as British households struggle with high gas prices, which have soared even more since the Russia-Ukraine conflict started in February. B…