Work with Accommodation

The following sites promote opportunities for work which includes accommodation:
http://www.indeed.co.uk/Free-Accommodation-Live-jobs-in-London
https://www.gumtree.com/jobs/london/accommodation
https://www.wwoof.org.uk/
http://helpstay.com/
https://www.workaway.info/
http://www.helpx.net
http://www.nomadwallet.com/free-accommodation-at-hostels-an-interview-with-a-die-hard-hostel-fan/
https://www.adzuna.co.uk/jobs/london/accommodation-provided
http://www.newsnow.co.uk/classifieds/jobs/live-in-accommodation-jobs-uk.html

Shared Accommodation

The following sites promote shared accommodation, such as Flat Shares:
http://www.spareroom.co.uk/
http://uk.easyroommate.com/south-east/london-flatshare
https://www.gumtree.com/flatshare/london/flat+share
http://www.moveflat.com/
https://www.roomhunters.co.uk/
http://flatshare.zoopla.co.uk/london-flatshare/
https://www.roombuddies.co.uk/

See also windows of local newsagents etc.

Low Cost and Free Accommodation

Low Cost Hostels. These can be booked online. Would suit those who can present as workers, visitors or tourists.
http://www.hostelworld.com
http://www.hostelbookers.com/hostels/england/london/
http://www.hostels.com/london/england
http://www.budgetplaces.com/london/budget-hostels-list/
http://www.travelstay.com/Cheap_London_Hostels.htm

Advice on getting Free Accommodation in London:
http://www.brokeinlondon.com/free-accommodation-london/