I think it's just the same as the Dengie SJing. You qualify for the Regional Finals, and then from them the National Finals, which are held at the Warks College, Moreton Morrell in April. Only for PC members, which I'm sure you are.
I presume you mean the Pony Club Dengie Dressage Winter League? It is an individual dressage competition (as opposed to a team event). First rounds are held by individual PC branches and the qualifying competitors from those go through to an Area Final. The qualifiers from the Area then go through to the National final in Moreton Morrell.
The first rounds are open to non Pony Club members, but if they qualify and want to go to the Area then they have to become Pony Club members.
My daughter did the Dengie last year and qualified for the Final and really enjoyed the experience.
Sounds like it - have you got a link to the schedule? You don't have to qualify through your own PC's competition, you can travel around and do as many first rounds as you like to try and gain a qualification. However, if you qualify you can only attend your local Area Final.
Like others have said it's the same as the Dengie jumping, the only difference (I think!) is that you can do Novice and Open at the regionals, the only thing to watch out for is that the tests get harder the Open one is much harder at Regional and Champs than qualifiers, so worth checking. I forgot to last year and ended up not going 'cause the test was so hard and I didn't start practising the peculiar movements - half canter 10m circles with a simple change early enough!