From the London Marathon website:

"When you negotiate the penultimate bend of the London Marathon, you will come face-to-face with Buckingham Palace and know that you have just 385 yards left until the Finish Line on The Mall.

The first edition of the race reached its climax just metres away on Constitution Hill, but from 1982 until 1993 the race ended on Westminster Bridge. However, in 1994 repair work to the bridge meant the Finish Line was moved to The Mall, where it has been ever since."