Eas a’ Chual Aluinn, pictured in the photo, is located near Glen Coul, Sutherland in the Scottish Highlands. With a sheer drop of 200 m (658 ft) it is highest waterfall in United Kingdom. When in full flow it is three times higher than Niagara Falls (Google Map).
Just beaten into second place is Cautley Spout (650 feet -198 m), in the Yorkshire Dales National Park, the highest waterfall in England. This is followed by Gaping Gill, the “highest” unbroken waterfall in England, with water falling 110 m from the surface into an underground cavern. The highest waterfall in Wales is Pistwll Rhaeadr (Clwyd) at a mere 240ft (73m).
Photo above from Geograph: © Copyright colin price and licensed for reuse a Creative Commons Licence.