Yes, I remember back-slang, sometimes called 'pig latin' and I don't think it was anything to do with Cockney. We also used a bit of Romani, from one grandfather, and bookees' slang from the other grandfather. I could do 'tic-tac' as well, but can only remember the 'evens' sign now.
My dad used the occasional polami word, that he picked up during WW2, and up to his death would give assent to something by saying 'bona'.
When my kids were teenagers, French kids were using 'verslen' a sort of backslang in which syllables were just reversed.
My DH hates any of these odd languages, and so I found myself dropping them around him, and only occasionally now use them with a few of my family. I still use a few of my native dialect words though.