In 1963, members of the Council of Europe, an intergovernmental human rights organization based in Strasbourg, adopted the Protocol No. 4 to the Convention for the Protection of Human Rights and Fundamental Freedoms. Article 1 of the protocol states that “no one shall be deprived of his liberty merely on the ground of inability to fulfil a contractual obligation.” Currently, 42 states have ratified the protocol.[74]
I got you
From [Wikipedia] (https://en.m.wikipedia.org/wiki/Debtors’_prison) about debtors prison:
Article about cash bail reform
I’m not sure if either of these apply to international flight risks.
Maybe a better option would be to seize British passports and place offenders on a no fly list.