Clearly, I have not seen your detailed legal advice, so I do not know, and I would have to reflect on whatever legal advice you received; we have not taken legal advice on the issue ourselves.
The one thing I would say is that, as you know, there will need to be a legislative consent motion on a number of the sentencing provisions and, off the top of my head, I suspect that this provision might be one of them. I know from discussions with the relevant Minister in Northern Ireland, who is publicly opposed to terror etc., that there are some very real practical concerns about extending this Act to Northern Ireland and some potentially unintended consequences.
So I think my answer to the question is this: listen very closely to your counterparts in the Department of Justice in Northern Ireland, and to the Justice Minister in Northern Ireland, as to whether this change is advantageous to the circumstances of Northern Ireland. Frankly, I cannot speak for either the Minister or officials, but I would take very careful cognisance of what they have to say to your own civil service colleagues.