Young Greenwich:
Universal youth services, includes lots of activities each week and special events (covering health, sports, activities/clubs, etc). This is open to all children and young people, not just siblings of children or young people with SEND. It is a wonderful resource and everything is free.

https://www.young-greenwich.org.uk/youth

Mencap:

https://www.greenwichmencap.org.uk/our-services/sibling-support/

Rosebuds Centre:

https://www.greenwichcommunitydirectory.org.uk/kb5/greenwich/directory/service.page?id=FhGwpJ6gO38&localofferchannel=0

Sibs: 

https://www.sibs.org.uk/