Get form B241 of the defence intranet. Fill it in sign it and give it to your chain of command.
They rules have changed significantly, which means they cannot stop it regardless of how short your particular arm or corps is.
After about 4 - 8 weeks you will have an answer and will be given an expedited transfer to the corp / arm of your choice. the Forces would prefer to let you transfer and kep yopu than see you sign off so it is now very very easy to do.