You cannot determine if a migrant is illegal until they are processed. Once processing is done, if the application fails, the applicant can be deported without problem (as has been happening a lot more recently owing to the increase in number of processors). The boats only started big time when the previous government closed all the 'safe routes' (where applicants were processed off shore) and reduced drastically the number of processors. This stupidity meant it took anything up to 3 years to process instead of a few weeks and led to putting applicants in closed hotels at huge expense. With faster processing and safe routes open, there would be no need to line the hoteliers pockets handsomely as happens now. The UK still needs people to work in the NHS, fruit & veg picking, care workers and hospitality staff - all traditionally done by migrant workers - and it needs fewer people believing that the UK can supply all its employment needs. It hasn't managed that for 5 centuries and idiotic and bigoted thinking won't change that.