In basic you will get found not guilty of any offences. You may still have to be interviewd at a station under caution to give your account.
Ultimately with UK law it is all about reasonable force.
It is all about circumstances around the incident itself and there will be a lot of "what if" questions regarding it but il give you a coupl of examples to answer your question.
If you are at home in bed at 3am And are awoken by noises downstairs which you think my be a burglar. You get up and go to check what is going on but before you do you get your RIF handgun from the storage cabinet in the next room just incase for self defence. You then go downstairs and see a burglar in your living room stealing yourstuff, you don't know what he is going to do or what weapons he has on him so you point your RIF at him telling him to stop what he is doing. You or your other half then phones the police who are making there way to you make sure you tell them you have a RIF pointing at the offender. The police arrive and take the burglar away they may also sieze your RIF as evidence and question you at a later date.
The other example is that you again are woken up at 3am by an intruder but rather then going to the cabinet/storage you keep your RIF next to you in bed and tell the police that you have it there incase a burglar is going to come into your house. This is seen as excessive as you have already pre-empted someone coming in meaning you already intend to us it and are keeping the RIF next to your bed for this reason.
Keeping any form of weapon next to you wether it be a gold club, bat or RIF for the reason of just incase someone comes in is classed as excessive.
So in recap yes you can use it as long as you keep it in your cabinet/storage and are using it for self defence only.
I know this is quite confusing but I hope it makes sense.