You can join the Army Air CORPS (not division) from other units of the Army but it's not easy to get in. You can also join direct if "lucky".
As for joining the RAF from the AAC - well, you can theoretically transfer service to service but in over a decade in the RAF I never met one person who came to us from the Army, although we had a few ex-sailor boys. The RAF likes its personnel to be able to read and write which excludes most soldiers!