but why doesn't CAA allocate 123.45 as a 'free for all' not allocated to any specific user?
As already stated the CAA does not allocate frequencies. As you say it would be a free for all and that is exactly what they want to avoid. If you have a genuine reason to use a radio in the Aeronautical Band then there is a process for doing so that insures you do not cause undue interference to other services and similarly nobody will cause undue interference to you. As with all such processes they cost money. If you are the holder of a radio licence and you are prosecuted for a radio offence then they have the right to withdraw that licence as the holder is considered to be a person unsuitable to hold such a licence.