A British Airways Airbus A321-200, registration G-MEDN performing flight BA-422 from London Heathrow,EN (UK) to Valencia,SP (Spain), was descending towards Valencia when white/grey smoke appeared in the cabin. The aircraft continued for a safe landing on Valencia's runway 12, vacated the runway onto high speed turn off H4 and stopped after crossing the hold short line. The aircraft was evacuated via slides. Three people were taken to a hospital. Passengers reported smoke appeared about 10 minutes prior to landing associated with a horrible smell, the aircraft was evacuated after landing. The airline reported the aircraft was involved in an incident, the aircraft has landed, passengers and crew have disembarked, British Airways teams are assisting the passengers.