Ok, well i dont know about you, but i cant seem to find the JLS fan mail address anywhere. But i do know how you can send them mail. You can send them fan mail to the venues they are performing at, at least a week before they perform.
Here is a list of venues in the U.K that do and dont pass on fan mail:

England
DO
-The Adelphi Theatre
-Albans Arena (St Albans)
-Almeida Theatre
-The Anvil Theatre
-The Apollo Theatre (London)
-Ashcroft Theatre (also known as Fairfield Halls) (Croydon)
-Assembly Hall Theatre
-Belfast Odyssey
-Birmingham Academy
-Birmingham Hippodrome
-Blenheim Palace
-Bournemouth International Centre (Possibility they will NOT pass on fanmail)
-Brixton Carling Academy
-Cambridge Corn Exchange
-Central Theatre (Chatham)
-Charter Theatre (Preston)
-Chichester Festival Theatre
-Churchill Theatre (Bromley)
-The Comedy Theatre (London)
-The Corn Exchange (Cambridge)
-The Courtyard Theatre (RSC)
-The Dominion Theatre
-Duke of York's Theatre
-The Friends Provident St. Mary's Stadium
-Garrick Theatre
-The Gielgud Theatre (London)
-Gordon Craig Theatre (Stevanage)
-Hampden Park
-Harewood House
-Hexagon Theatre (Reading)
-KC Stadium
-Komedia Bath
-Komedia Brighton
-Leeds Cockpit
-Liverpool Empire Theatre
-London Palladium
-The Lowry, Salford
-The Lyric Theatre (London)
-Manchester Apollo
-Marlowe Theatre (Canterbury)
-MEN arena
-The Menier Theatre
-The Milfield Theatre (London)
-Milton Keynes Theatre
-The National Theatre
-New Victoria Theatre (Woking)
-New Wimbledon Theatre
-NIA (Birmingham)
-Nottingham Royal Centre
-O2 Arena (formally the Millenium Dome)
-The Palace Theatre (London)
-Playhouse (Western Super Mare)
-The Playhouse Theatre
-Rang Plymouth Pavillions
-Ricoh Arena
-Royal Court Jerwood Theatre Upstairs
-The Sage (Gateshead)
-The Savoy Theatre
-Sheffield City Hall
-Sheffield Arena
-Shepherds Bush Empire
-Southampton Guildhall
-Theatre Royal, Newcastle
-Theatre Royal Drury Lane
-The Trafalgar Studios
-The Wycombe Swan Theatre
-Wyndhams Theatre
-Wembley Arena
-The Young Vic Theatre
-The Sands Centre (Carlisle)
-The Royal Albert Hall


DON'T
-Brighton Centre
-Cardiff International Arena
-Hammersmith Apollo
-LG Arena *formerly known as NEC Arena*
-Liverpool Echo Arena
-Roundhouse (Camden, London)
-Wembley Stadium

[ie] Ireland [ie]
DO
-Odyssey Arena
-RDS Arena
-The Point

Scotland
DO
-Aberdeen Exhibition & Conference Centre
-Kings Theatre
-Queens Hall
-Glasgow Royal Concert Hall
-Glasgow SECC

DON'T
-The Liquid Room

If you know anywhere else that does please let me know.
Thanks, and i hope this helps x