South London Symphonic Winds a community wind band playing a range of music from classical arrangements to 21st Century commissions.
South London Jazz Orchestra a community jazz big band which is open to local people wanting to play Big Band music.
BASBWE British Association of Symphonic Bands and Wind Ensembles, supports and promotes symphonic wind bands.
Making Music represents and supports the development of amateur orchestras, bands, and choirs throughout the UK.
Arts Council (London region) develop and promote the arts.
UK Amateur Orchestras list of non-professional orchestras in the UK, searchable by region.