The cost of landing at international airports is what puts most people off.
Manston is one of the biggest runways in the UK, so should satisfy some urges.
Without question, Lashenden (Headcorn) is the friendliest GA field in the SE and my favourite field to fly to. Jamie is always helpful and cheerful, always plenty going on and always some nice aircraft to see. Good for lunch too.
Southend is also good and both of these two are good points to start a channel crossing from.
Add North Weald to your list too. Again very friendly, cheap landing fees and good bacon sarnies. Jets to watch too. In fact, go there while you can, as its the subject of a planning inquiry at the moment and we may lose it forever.