"Also, there's no age limit on the defence for modification. A 13 year old can be gifted an IF and modify it into a RIF for airsoft use."
Er, no thats manufacturing a RIF Illegal, you need a defence to do it. 13 or 31 doesn't matter, it is illegal.
You need to either:
1. Talk you dad into playing.
2. Use hire guns till your old enough to buy.
3. Get a two Tone.
4. Talk to site shop see if they will sell you one, but they store it at the site for you.
They are the immediate options In the longer term you get a two tone and establish yourself at the site till you have a membership ( if there insurance allows 13 year old members ). At this point as a skirmisher you have a defence and can then manufacturer a RIF, ie paint your two tone. UKARA is irrelevant, its basically a manufacturers Association. Having your club membership ( your defence ) registered on there database makes it easier to prove your defence but that is all. Far from being a licence it isn't even a defence.
Final interesting point eve with a defence it is still illegal to Sell, Import or Manufacture RIF's. We are all braing the law. we simply have been granted a defence from prosecution. Two facts about this are scary. it has never been tried ncourt so until it is, who knows what could actually happen. Second worry is the Home Seretary can at any point without giving a reason or an act of parliment revoke that defence. That is why we all have to be very carefull what we do. On major incident like boyo on the roof or the lads n the field and airsoft could be gone.