You need defence when you want to import (bring from abroad) your RIF. Defence can be UKARA but if you can prove you are an airsofter, that's okay too. For example if you are a registered member of an airsoft club, and you have the paperwork, that should work - if they can check it somehow. Or you have booked for an airsoft event here in the UK and the site can confirm it, that should also work. So you can prove you are an airsofter without UKARA but it's harder. And if that doesn't work out, the rifs will be destroyed.
Once your RIF is here, you don't need anything, it's legal to own them.
As for the other equipment: if those are legal to own and import to the UK, it should be ok.
When I moved here, I two-toned my gun by changing the stock and the frontset to some cheap ones painted bright green. I also brought the normal parts. Then I booked for an airsoft event, and switched them to the proper ones on site (manufactured RIF, but since I was at the airsoft event, that was my defence). Switched back to IF at the end of the game. I did that 3 times got my UKARA number, and imported the rest of my rifs.