I think a common misconception is that UKARA is not a licence or a law, it's just a database. That's all it is.
The law, or rather the statute, is that to own realistic weapons you're to be able to prove that you're a regular skirmisher, the statute does not describe what 'regular' is but the home office (or so I'm told) gives 3 times in less than 2 months a guideline. If you're a regular skirmisher, and your site owner can attest to that then you'll have no problem with painting your gun.