As others have said, we need to deter people from wanting to come to the UK in the first place.
Make it harder for people to find "black economy" work by tackling the black economy, i.e. illegal working, modern slavery, tax evasion, benefit fraud etc.
Compulsory biometric ID cards so the law enforcement authorities know who they're dealing with to stop people "sharing" ID documents or slipping away avoiding court/bail etc after being arrested and released using false/fake ID.
Massive clamp down on money laundering via Turkish barbers, nail bars, ethnic food speciality shops, mobile phone accessory shops, dodgy takeaways etc., that never seem to have any customers. Clamp down on modern slavery in hand car washes, food processing factories (abatoirs), the scrap metal and other trades, etc.
To an extent, that's probably part of what Rishi was hoping to do with his Rwanda plan - get the message across that "irregular" immigrants would get sent somewhere like that, and of course, it solved the problem often cited of other countries refusing to accept them back - Rwanda wanted them! But of course, like most things Rishi did, he screwed it up!
Labour's support is collapsing in previous "red wall" Northern areas because they're relocating these irregular immigrants into Northern towns and cities, but not putting in any support or law enforcement infrastructure, so crime is rising, anti social behaviour is rising, etc. These are places already suffering from London Centric policies and feel they're being kicked when they're down by large influxes of "problematic" people. It was bad enough when councils of run down seaside towns embraced the likes of released prisoners to help fill their empty guest houses that caused massive social and crime problems, but it's a kick in the teeth for some of those same places, having dozens/hundreds of illegal immigrants being bussed in as well.