I run an education recruitment business and that’s not true.
You can’t legally be classed as a teacher in this country without a recognised UK qualification (QTS or QTLS).
We do have overseas Cover Supervisors. Most of them are in the country on student visas, studying Masters level qualifications in UK universities and working alongside those studies.
They generally have taught in their country of origin, or are from professional backgrounds like engineering etc. Generally very qualified people, who just don’t have the relevant qualifications to be a recognised teacher here.
They’re not invited to teach, they arrive on other visas (usually student or sponsored) and work to supplement their income.
We do have a teaching retention and recruitment crisis (for the reasons others have said) and those gaps are filled with agency staff, who also don’t have teachers coming out of their ears, so offer cover staff - some of whom may be from overseas.