UK domestic law says these arrivals are illegal but the UK is a signatory to the 1951 Refugee Convention, which provides certain protections:

-Article 31 of the Convention states that refugees should not be penalized for their illegal entry or presence, provided they present themselves to authorities without delay and show "good cause" for their entry. (Yeah, I know...)

- Under international law, anyone has the right to apply for asylum in a country they have reached. Once a person makes an asylum claim, they generally have a legal right to remain in the UK while their application is processed.

- If a person's asylum claim is granted, they are recognized as a refugee, and their initial "illegal" entry/status is effectively regularized.

https://www.channel4.com/news/factcheck/factcheck-is-it-illegal-to-cross-the-channel-on-a-small-boat