London Walks are as usual on hand with a few such walks, with the Ghostly Old City on Halloween leaving at 7 pm from St Paul's tube station, exit 2, Haunted London on Halloween leaving from Monument at 7.30 pm, Ghosts, Gaslight & Guinness leaving at 7.30 pm from Holborn and Apparitions, Alleyways & Ale leaving from 8pm from Embankment. Your author has only previously done the Apparitions, Alleyways and Ale walk, which was pleasantly spooky - especially the jaunt through St James' Park.
For more, see http://www.walks.com/London_Walks_Home/HALLOWEEN_WALKS/default.aspx