Lots of paperwork and some money needed
Where the facilities are available - the Government made a big song and dance that they were 'establishing an office to process claims in France' that turned out to be one desk in an airport - one that wasn't manned most of the time..
The whole focus is wrong. To deter illegal migration, you need to offer legal options that the people who want to migrate can access. If you don't, people turn to the people smugglers (they are the people that the home office should be going after, not the migrants!) and the whole cycle turns again, with more people dying in overcrowded fishing boats just so that some criminal can make lots of cash.
Given fair legal options, the 'illegal' migrant number will drastically reduce. But this (or any UK) government doesn't want to do that because the same people who voted for Brexit will be manipulated into frothing hatred of migrants by the likes of Priti Patel, Braverman and Johnson (who, ironically, are all descendants of immigrants!) and the government doesn't want people unhappy enough to vote for any alternative.