Back to the OP.
Unless your mother can provide the seller with a defence against conviction under the VCRA then it is illegal for her to buy an RIF. If she CAN provide a defence, then it is perfectly legal for her to GIVE it to you AS A GIFT. NO money can change hands IN ANY WAY in respect of the gun. You can't pay your mum back for it, it has to be a gift. It is entirely illegal to SELL any kind of immitation firearm to a minor.
Now back to what I think you were actually asking.
Is it illegal for your dad to assert ownership of the gun, and to state that he will remove it if you abuse it, no, there is nothing illegal about that. You might think he's being strict, but look at it from his point of view. If you decide to wander off with a realistic looking firearm tucked down the back of your trousers, the chances are you will end up in a lot of trouble. Possibly facing down a squad of heavily armed coppers with very real, very loaded, very deadly MP5's. One misunderstanding could end up with you meeting large chunks of lead with a very high closing velocity.
So, ask me again if there is anything illegal about his attitude.
His attitude reflects a responsible, adult attitude to allowing a young person access to guns. He'd rather not have to identify you on the coroners slab because you've done something silly!
Sound melodramatic? Maybe, but it's happened before and it will happen again. At least he'll allow you to have the weapon in the first place, most parents are so gun-phobic that they won't even buy plastic water pistols for their kids, in case they somehow turn them in to crazed killers.
)