The exact centre of London
At the corner of the Strand and Charing Cross Road.
The exact center of London is marked by a plaque in the Church of St. Martin's-in-the-Fields overlooking Trafalgar Square.
The actual point is on the corner of the Strand and Charing Cross Road, near the statue of Charles I, there is even a plaque on the wall confirming this. The statue marks the spot of the original Charing Cross.