If you fail to report a notifiable acquisition of shares that provides control of a relevant entity and where there is a connection to the UK, then the transaction is void and you may commit a criminal offence and be liable to fines. As a result, you must go through the clearance process and you must know what to look out for.
A transaction must be notified if the shares to be sold confer a defined level of control on the buyer and the entity trades within any of the 17 sensitive sectors. Alexandra's article sets out more detail as to both tests. If these tests are met, the deal must be notified.
If these tests are not met, then the buyer may still choose to notify, as the Secretary of State has five years in which to investigate and possibly void the transaction. The Secretary of State will use that power where, in any sense, National Security is at stake. See "Assessing asset deals" below.