Hypothesise:
This becomes possible, legal, enforceable, whatever, on Monday.
How many generations back does a non-United Kingdom born ancestor trigger you to be sent back?
How many ancestors, and how far back, is required to trigger the provision? For example, is one great-grandparent enough? Two? Does two great-grandparents equal one foreign grandparent?
Is an ancestor born within the United Kingdom sufficient, or do they need to be born within a specific country of the United Kingdom? Will the English be deported from Scotland?
If the country of the ancestor was a Crown possession, or Crown dependency, at the time, does that count as foreign?
Can you be sent back even if you have no entitlement to citizenship of the country you are being sent back to? (Currently this is a violation of international law and comity, but it's an open option to decide it is worth becoming an international pariah state, this is a hypothetical situation after all)
Is it ever possible to 'become' a UK citizen with any non-UK ancestry? Again, how far back will you trace a family line? Obviously not back to medieval times.
Will DNA testing be employed to check the racial makeup of UK citizens?
What methods will be used to verify family history where birth records are untraceable in UK registers?