I would describe them as a person seeking asylum.
There are what our government consider “illegal routes” and what it consider “safe and legal routes”.
But how would any person seeking asylum get to the UK if they don’t fit into one of our governments tiny boxes of people they apply the safe and legal routes to? So people will take unsafe routes to the country, like the ways you say.
Many people seeking asylum don’t have passports so can’t get to our passport control as we are an island.
Once in this country, the first thing they will do is claim asylum, and at which point, they are documented and not illegal. Even if their claim and subsequent appeal is rejected, they are then still not illegal even though they have no recourse to public funds and will be destitute and homeless (which the government actually allows and rather than deporting them after their claim fails, makes them check in monthly at a police station etc, but doesn’t actually support them at all).