What would happen if the UK had an In/Out referendum?

Daniel Hannan in The Telegraph asks the question and comes to this conclusion:

'The House of Commons is there to represent and defend the interests of the people. Parliamentary sovereignty is an expression of popular sovereignty. Our MPs are the beneficiaries and exemplars of a centuries-long struggle to vest supreme authority in the hands of elected representatives. If they wish to surrender a measure of that authority, they ought at least to ask our permission.

If the referendum goes in favour of continued membership, I promise to accept the result with as much good grace as I can muster. The issue will be settled, the long defeat complete. But let’s at least put the question.'

It is odd how most of those who would vote 'Out' in any referendum would accept whatever the result was and how so few of those who would vote 'In' would do so. Is this anything to do with the likely result or the respective side's views on respecting democracy?