How to confirm "confirmed onward flight that leaves on the day you arrive or before midnight on the day after you arrive"?

Rules for transit in the UK for Indian citizens
You’ll need a visa to pass through the UK in transit
You should apply for a Visitor in Transit visa if you arrive on a flight and will pass through immigration control before you leave the UK.
Transiting without a visa
You might be eligible for ‘transit without visa’ if:
you arrive and depart by air
have a confirmed onward flight that leaves on the day you arrive or before midnight on the day after you arrive
have the right documents for your destination (eg a visa for that country)
How to confirm the second point?
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