So why don't we process everybody in the same way? In what way do refugees, asylum seekers, and economic migrants pose a greater threat?

The legal status of refugees has been dealt with ad nauseam. Asylum seekers have no way of reaching the UK without making prior landfall outside their own country.Therefore their legal status should have been already established. Economic migrants, unless EU citizens, have to satisfy the necessary criteria before being allowed entry. Everyone is processed coming in by ship or air. Unless aliens are processed how can a potential threat be recognised. Even with stringent vetting and profiling some dodgy people can enter.
If I have to explain why aliens can pose a potential threat I do believe further debate is futile. Do we not have a sufficiency of home grown terrorists? Research by The Soufan Centre estimated that at least 425 British Isis members have so far returned to the UK – the largest cohort in Europe.
But there are fears many have “disappeared” from the view of security services, who will not publicly confirm how many returnees have been jailed or are being tracked.