Great question — answering as the UphillTech team:
I am retired but I used to be an Immigration Officer for what is now UK Border Force. The onward booking rule was often a requirement of transit visa waivers. So a national of certain countries would normally require a visa for a visit but if they were in direct transit within 24 hours there were visa waivers, and could be admitted without a visa, as long as you had a confirmed onward flight. So folk up to no good could try to exploit this to get around the visa requirement. (Usually folk whose circumstances meant they were unlikely to qualify for a tourist visa). In the UK we routinely asked for evidence of onward travel. You then looked at whatever the passenger produced and made a...
Full answer by Elwyn Soutter: https://travel.stackexchange.com/a/203658 (CC BY-SA 4.0).
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