Entrance to the bunker apparently housing the terminal equipment of the BT cross-channel cables at St Margarets bay near Dover.
I could not locate this, until a reader of this blog pointed me to a large building in Lighthouse Road. You can see this view through the gate:
Here is the main building, adjacent to the image taken above:
Here is some aerial views from Bing maps.
It looks like the area is being developed at present (Dover District Council planning approval 05/01014 Erection of a new ‘top man’ station building (existing below ground staircase enclosure to be demolished) incorporating existing stairs and access well, two offices and w.c. | Part of BT Exchange, St Margarets Road, St. Margaret’s Bay).