There are many ways the UK can control, opt-out or veto the EU’s decisions in regards to asylum seekers, immigration and criminal law.
The UK will not be covered by the proposed law on a European Border Guard. While this law originally provided for the border force to enter a Member State without its consent, that idea was dropped during negotiations. That would anyway not have applied to the UK; and in fact the EU court has ruled that the UK could not opt in to the EU law creating a border agency (the precursor to the proposed Border Guard law) even if it wanted to, without signing up to the whole of the Schengen system.
In the areas of criminal law and policing (which will be the subject of a separate blog post with more detail), the UK had a veto until 2009, when the Treaty of Lisbon came into force. Since that date, it has had an opt-out, which it has frequently used. In particular, it has opted out of the proposal for a European Public Prosecutor.
Note that the EU’s police agency, Europol, is not a ‘federal police force’: the Treaty rules out ‘coercive powers’ for it, so it cannot arrest, question or detain people. Its main role is the analysis of police investigation data.
The abolition of the opt-outs on immigration and asylum, Schengen and criminal law would require a Treaty amendment subject to approval of the government and Parliament. The abolition of the Schengen opt-out would also require a national referendum, under the European Union Act 2011. So would participation in the European Public Prosecutor.