It will be a little egret........which is becoming fairly common in the UK. As long as there is ice free water, likely to be rivers, then it should be ok. In general most water birds tend to shif to coastal estuaries in weather like this. Most need ice-free water to feed on and it just is not there at the moment. I am a volunteer warden at RSPB Otmoor near Oxford. Waterfowl numbers are usually very good in winter. At the moment though numbers are extremely low....there is very little ice free water and I would suspect most have decamped to the coast