Advertise it for sale, see if a local school would like it or donate it to a charity. The following charity shops will accept musical instruments in good condition: Prospect Hospice, Helen and Douglas House, RSPCA and Blue Cross; all in Marlborough High Street. Check with them first if it is a large instrument. Sell on ebay or give away on freecycle.
Rather than throw an old instrument away, why not have it repaired and enjoy it again?
Geoff Kempster, of Kempster Music in Commercial Road, Swindon, does some guitar repairs and can advise on other repairers who specialise in specific instruments. 01793 535523
Recycle metal instruments, that cannot be repaired, in the metal container at Marlborough Household Recycling Centre.