The central inspiration for Pagan London stems from the lifelong work of Ronald Hutton, professor of history at the University of Bristol, whose books are a joy to read, combining forensic attention to detail with a palpable, genuine love for the subject. Buy them.

Peter Ackroyd, London: The Biography (Vintage, London, 2000)
Miranda Green, The Gods of the Celts (Alan Sutton, Gloucester, 1986)
Ronald Hutton, Stations of the Sun: A History of the Ritual Year in Britain (Oxford University Press, Oxford, 2001, first published 1996)
Ronald Hutton, Blood and Mistletoe: the History of the Druids in Britain (Yale University Press, London, 2011)
Ronald Hutton, Pagan Britain (Yale University Press, London, 2013)

Stanwell Cursus and Shepperton Henge
The Archaeology at Heathrow Terminal 5 website is a great place to explore.
Rodney Castleden, Neolithic Britain: New Stone Age sites of England, Scotland and Wales (Routledge, Abingdon, 2015, first published 1992)
Gordon Barclay and Gordon S. Maxwell, The Cleaven Dyke and Littleour: Monuments in the Neolithic of Tayside (Society of Antiquaries of Scotland, Edinburgh, 1998)
Vicki Cummings, The Neolithic of Britain and Ireland (Routledge, Abingdon, 2017)
Paul Devereux, Spirit Roads: An Exploration of Otherworldly Routes (Collins & Brown, London, 2007, first published 2003)
Jim Grant, Sam Gorin and Neil Fleming, The Archaeology Coursebook: An Introduction to Themes, Sites, Methods and Skills (Routledge, Abingdon, 2008, first published 2001)
Lamont Lindstrom, ‘Cargo Cult at the Third Millenium’, in Holger Jebens (ed.) Cargo, Cult, and Culture Critique (University of Hawai’i Press, Honolulu, 2004)

The Dagenham Idol
Bryony Coles, ‘Anthropomorphic Wooden Figures from Britain’, quoted in Marion Gibson, Imagining the Pagan Past: Gods and Goddesses in Literature and History since the Dark Ages (Routledge, Abingdon, 2013)

Fenning’s Wharf
J. B. Burland, J. R. Standing, F. M. Jardine (eds.), Building Response to Tunnelling: Case Studies from Construction of the Jubilee Line Extension, London, Volume 1 (Thomas Telford, London, 2001)
Journal of the Royal Statistical Society, Volume 1 (Charles Knight, London, 1838)
The map is from Jennifer Proctor and Barry Bishop, ‘Prehistoric and environmental development on Horsleydown: excavations at 1-2 Three Oak Lane’, in Surrey Archaeological Collections, 89, 1-26, 2002
Bruce Watson and Ian Tyers, ‘Wood and Water or Rebuilding Medieval London Bridge’, in E. Shepherd (ed.), Interpreting Stratigraphy 5 (University of York, Norwich, 1995)
The History of London-Bridge: From its First Foundation in the Year 994, to the Destruction of the Temporary Bridge by Fire, on the Eleventh Day of April 1758 (M. Cooper, London, 1758)

Angela Boyle and Robert Early, Excavations at Springhead Roman Town, Southfleet, Kent (Oxford Archaeological Unit Occasional Paper Number 1, 1994?)
Barry C. Burnham and John Wacher, The Small Towns of Roman Britain (University of California Press, Berkeley, 1990)
Dorothy Watts, Christians and Pagans in Roman Britain (Routledge, Abingdon, 2014, first published 1991)
Eileen M. Murphy (ed.), Deviant Burial in the Archaeological Record (Oxbow, Oxford, 2008)
Jessica M. Grimm, ‘A bird for all occasions: the use of birds at the Romano-British sanctuary of Springhead, Kent (UK)’, in W. Prummel, J.T. Zeiler and D.C. Brinkhuizen (eds.), Birds in Archaeology: Proceedings of the 6th Meeting of the ICAZ Bird Working Group in Groningen (Barkhuis, Groningen, 2010)
Martin Millett, Louise Revell, Alison Moore (eds.), The Oxford Handbook of Roman Britain (Oxford University Press, Oxford, 2016)

The Temple of Diana
John Clark, ‘The Temple of Diana’, from Interpreting Roman London: Papers in Memory of Hugh Chapman (Museum of London, 1996)

The Maypole on the Strand
John Brand, Maydays, Maypoles, and Morris Dancing (Folklore History Series) (Pierides Press, 2010)

The Obelisk of Ra
Agnieszka Dobrowolska and Jarosław Dobrowolski, Heliopolis: Rebirth of the City of the Sun (The American University in Cairo Press, Cairo, 2006)
N.B. As astute viewers have pointed out, the clock tower is not on the Savoy Hotel but on the Shell-Mex building next door. Fake news. Sad!