This petition was rejected

The Government e-Petitions Team gave the following reason:

Immigration officers already have the power to take several actions against illegal migrants, including the power to detain them for "the purpose of removal": assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/537231/Immigration_Act_-_Part_3_-_Enforcement.pdf.

The Home Secretary must already make a deportation order in respect of a foreign criminal unless certain exceptions apply (e.g. where deportation would contravene the UK’s obligations under the Refugee and Human Rights Conventions). You can find out more here: commonslibrary.parliament.uk/research-briefings/cbp-8062/

Further, not all undocumented migrants entering the UK are doing so illegally - many will be entering the country for the purpose of seeking asylum: www.gov.uk/claim-asylum.