This 'early activity' is not down to global warming as some would have it; Rooks always stake their claims in the winter months. Perhaps such actions give time for further dispersal by the unsuccessful? Perhaps sorting rank and position in a colony takes time that they can ill-afford in the true breeding season? Whatever the reason, their return to the nests is another promise of Spring.
(If anyone comes across it, there is a lovely old book, "The Life of the Rook", by G.K. Yeates. First published in 1934, it describes their nesting habits in great detail, based on observations made just down the road in East Kent. It carries a lovely dedication to 'The George and Dragon' at Fordwich and all its inmates.)