The tallest yew hedge in Britain is apparecntly the 40ft one on the Bathurst Estate in Cirencester, Gloucestershire. (Obviously there are hedges of leylandii and beech, amongst other species, that are a good deal higher).
There’s a good picture of it in this article.
These are screengrabs of the view from the outside:
The hedge mainly seems designed to separate the Batshurst estate (home to Lord Apsley) from the town centre – you can see on the map below that the estate cuts right into the town (the hedge is under the word “Museum”).
Of course leylandii hedges of these dimensions often cause bitter arguments between neighbours, but I suspect the town council won’t be invoking the high hedges legislation and asking Lord Apsley to chop his yews down to size any time soon.
(Thanks to DrMr for pointing me towards this one.)