The majority of the flooding in Gloucestershire and Oxfordshire is nothing to do with the water companies. Just because they are responsible (or are supposed to be responsible) for surface drainage from houses and highways doesn't make them responsible for the safe passage of ALL excess water from the place it falls back to the sea. Over 95% of the water that fell landed in non-urban areas, trickled into streams, then rivers, then swamped the Avon, Thames and other major rivers. Sure, some localised flash flooding in towns came from overwhelmed storm drains and that caused local problems and a small number of cases of flooding which quickly receded.
The main 'extra costs' incurred by the water companies are for overtime for the valiant water workers who helped out, and for the bowser / bottled water fiasco. That cost should come out of company profits (from shareholders). But I suspect your question also relates to costs of wider infrastructure repair caused by mass flooding of river plains.
It's an Act of God made worse by man building on the flood plains, leaving the water fewer places to safely overspill into.