Mandatory embarkation checks (checking those leaving the country) were partially removed by the Conservatives in 1994 and scrapped altogether when Labour came to power.
Once Labour got in they rapidly allowed the UK to become a haven for economic migrants, bogus asylum seekers, criminals, extremists, general scum of the earth etc due to their total lack of control and their immigration policy being aimed at creating millions of future labour voters.
Other than spouses and key workers the current administration should put a total ban on all immigration until the mess is sorted out.