You are.I know I'm clutching at straws!!!!!
Firstly I don't see how you could fuse down part of a ring in the way that you describe - if I've read it right you have this:
Secondly, you can't run the whole ring on a 20A breaker - the only ring final permitted by the regulations is one with a 30/32A device and cable with a minimum size of 2.5mm² and a minimum capacity of 20A.