What's going to happen is when you call up for taxi with the usual persons onboard, outbound to your destination, bla, bla, the controller then knows which way you'll be routing. So on departure from 21, he'll probably say "cleared for takeoff, right turn". Just as you've finished your turn he will probably ask you "G-XXXX, which altitude are you climbing to?". And after that he'll say "report Ashford", or Rye, Tenterden or Folkestone or wherever makes sense with your routing. Simple as that.
Lydd is a very friendly field with good controllers. Only gotcha is the two restricted areas over the power station and the firing range. They're not always active, but it will say so on the ATIS.