This wire was originally known as 'Rendall's washing line'. BOAC's Captain 'Flaps' Rendall was concerned that there was nothing with which to assess the ac attitude when making a visual approach, so he had this wire added after one of the early route proving flights.
Interesting historical note - 'Flaps' flew the first ever Standard VC10 revenue flight in April 1964, his son flew them wiht BOAC and BA and his grand daughter Lucy captained the last ever flight of a Standard VC10 in March 2001 when the RAF's last VC10K2 was flown to the knacker's yard at St. Athan. Where she flew a very nice approach in a stiff cross-wind onto the short wet runway.....