Originally Posted by
corporate-wage-slave
Canada, Bermuda, Caribbean or Mexico will do, so probably something like an Avios booking from NYC to YYZ. Avios bookings are very flexible, and can be altered with very little expense.
If you're out of Avios, then some airlines do cheap singles, such as Norwegian, Azores, Icelandair.
I would be wary of telling absolute porkies to the CBP, it isn't likely to end well. But saying "I have this booked on x June, but I hope to leave before then" would be fine.
If travelling on an ESTA be VERY careful about that. I can only assume an ESTA for UK citizens has the same rules as for an Australian (??) and I can tell you that for Aussies going to Canada/Mexico etc does NOT constitute "leaving" for ESTA purposes. People have mainly tried that on to stay LONGER than the ESTAs 90 day allowance... and been caught when trying to reenter the USA.. however I suspect the same strictures apply for proof of departure...