Make sure it is your second choice as if you accidentally put it as your first choice you will go there no matter what.
That is only if you meet the requirements for your insurance (2nd choice).
e.g lets say your first choice requires AAA and you get ACC at the end of A levels. And your insurance choice requires AAB, then you can't get into any as you got lower that both your firm and insurance. But you can still go into clearing where you phone other universities on results day to get a place. (I think that is what happens, I haven'tread too much on it)
But in your case you have an unconditional offer. So lets say your firm choice needs AAA and your second choice is unconditional and you get ABB, as you did not get the required grades for your firm choice you will 100% go to your second choice as it is unconditional.
So make sure you like the universities that you will be firming. Especially the university that gave you unconditional offer.