London is split into North and South by the river Thames which runs through the centre.
Here's a guide to which suburbs are north and south of this natural divide.

North of river Acton Barking Barnet Beckton Becontree Brent Brentford Chadwell Heath Chelsea Chingford Chiswick City Clapton Collier Row Cricklewood Dagenham Dalston Dollis Hill Ealing East Acton East Ham Edgware Edmonton Enfield Feltham Finchley Forest Gate Fulham Greenford Hackney Hammersmith Hampstead Hanwell Haringey Harlesden Harrow Hatton Havering Hayes Hendon Heston Hillingdon Hornsey Hounslow Ilford Isle of Dogs Isleworth Islington Kensington Kenton Kilburn Kingsbury Leyton Leytonstone Lower Edmonton Manor House Manor park Mill Hill Muswell Hill Neasden New Southgate Newham Northwood Notting Hill Osterley Paddington Palmers Green Plaistow Poplar Redbridge Romford Ruislip Shepherds Bush South Woodford Southall Southgate Stanmore Stepney Stoke Newington Stratford Teddington Tottenham Turnpike Lane Twickenham Upper Edmonton Upper Holloway Uxbridge Waltham Forest Walthamstow Wanstead Watford Wembley West Acton West Ealing West Ham Whetstone Whitechapel Willesden Winchmore Hill Wood Green Woodford

South of river Abbeywood Addington Barnes Battersea Beckenham Bermondsey Bexley Blackheath Brixton Brockley Bromley Camberwell Carshalton Catford Charlton Chislehurst Clapham Colliers Wood Croydon Crystal Palace Dartford Deptford Dulwich East Dulwich Eltham Forest Hill Greenwich Ham Hatton Hayes Heston Kew Kidbrooke Kingston Upon Thames Leigh Green Lewisham Merton Mitcham Morden Mottingham New Cross New Malden Osterley Penge Putney Raynes Park Richmond Rotherhithe Sidcup South Norwood Stockwell Streatham Surbiton Surrey Quays Sutton Sydenham Thamesmead Vauxhall Wallington Wandsworth West Norwood West Wickham Wimbledon Woolwich