Actually that is not strictly true, Ichkeria.
There is nothing to prevent a government passing legislation to see the UK's witdrawal from the EU. Nothing in statute suggests that it can only be done following a favourable referendum. As with all other legislation, it does not even have to be included in a party's manifesto (history is littered with legislation about which the electorate knew nothing of the proposals until a bill is introduced in the Commons. The most recent I can think of is the same-sex Marriage Act). Parliament is supreme is such matters and all that is needed is for the measure to pass successfully through the Commons and Lords.
I accept that no party capable of forming a government is remotely likely to introduce such a measure, but there is nothing technically to prevent it (unless anyone can point me to something which contradicts this !).