Assets deals are not subject to mandatory notification, however where the Secretary of State wishes to investigate the matter, the deal can be unwound within five years of completion. The only way to avoid this is to notify and get clearance.
You will need to assess the risk of there being a national security element to your asset deal. A link to the 17 sensitive sectors is the most likely indicator, but you can't ignore other national security factors. As above, Alexandra can offer a fixed-fee service to advise that the asset deal is unlikely to be "called in" by the Secretary of State. However, in reality, you are probably best just phoning her to discuss the asset deal. It will be rare for a normal' asset deal to be caught. (A normal asset deal, would not include a deal that (but for a clever pre-sale restructure to avoid the NSI Act) would have been a share deal and would have been caught as such.)