I have never found a place in the U.K. where you can get close enough to cormorants for photography from a public hide. There is a boating pool in Helston, Cornwall where you can walk close enough to them on a small island, but the background is not great. In Dorset at Poole Park I have taken images occasionally on foot, but again the settings are not great. This is taken in Romania at the Ultima Frontiera Reserve. Canon EOS 7D Mk2. 100-400mm lens at 340mm. 1/400th at f11 with 400 iso. The hide has an opening at floor level so you can lie down very close to the water.
Great cormorant, Phalacrocorax carbo, single bird on post in water, Romania, June 2016