If you want to obtain a UKARA registration, which is actually just a site membership number, you have to play 3 games at that site over a minimum of two months, e.g. the third game being 56 days or more after the first.
There is no other way of doing it.
UKARA registration only provides a defence relating to airsoft toys, not to any other form of imitation firearms.
You probably should have checked the rules on importing imitation firearms before you ordered one from overseas.