Not from the airports there aren't; indeed, there aren't even National Express buses from neither LHR nor LGW to Canterbury, except via London Victoria. Likewise, all trains go via London or other places, such as Tonbridge. By train, it'll take 2-2.5 hours from LGW; about 2.5 hours from LHR (using the Tube to London Victoria or Charing Cross); about 2.5-2.75 hours from LTN Parkway, normally via London Bridge and Tonbridge; and about 3-3.5 hours from STN (Stansted Express to London Liverpool Street, thence Tube to Charing Cross, thence train to Canterbury). So, time-wise, it doesn't make a huge amount of difference into which London airport you fly. For times, prices, etc., visit www.thetrainline.co.uk.
[Edited 2006-07-29 16:32:31]
"Everyone writing for the Telegraph knows that the way to grab eyeballs is with Ryanair and/or sex."