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
Category: Accommodation
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.
Shared Lives
Site which matches those with Support Needs with those with Housing Needs who can provide Support:
http://sharedlivesplus.org.uk/about-shared-lives-plus/home-share
Camp Sites
If you have the use of camping equipment, the following may be of use:
http://www.visitlondon.com/where-to-stay/caravan-and-camping#Qhw0JJ7cySMJrmga.97
http://www.ukcampsite.co.uk/sites/results.asp?county=London
https://www.coolcamping.co.uk/campsites/uk/england/near-london
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.northfieldshostel.com/
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/