Is there any way of finding out what affixes are allready spoken for? I have chosen one but not sure if it exists allready, is there a central registry you can check first before applying or do you just apply and the kennel club let you know?

The Kennel Club publish a handbook of all registered affixes. If you get a copy you would know what words are unavailable.

I have found that if you use the contact us form on the KC website and select affixes from the topic area, then they are usually quite good about responding back about whether an affix has been used or might be rejected within a day or so