One cannot trample slipshod over the unwritten British Constitutional laws of the House of Commons. Elected MPs will always use their powers of a free vote, to uphold their rights in any such circumstances ! Party Whips know they could never prevent this.
Theresa May, the Prime Minister, must also respect this. "Brexit or not" is a perfect and typical example of when she will be expected to ensure a free vote by MPs, BEFORE she actions Section 50 of the Lisbon Treaty to start any official "Brexit" negotiations !
Since the "Brexit" vote by Electors in the Referendum, many other events have also forced EU leaders to recognise, that changes in the EU are now urgent, even if only to ensure its future !
This has been confirmed by Angela Merkel, who faces a General Election in Germany next year, just like François Hollande in France. Both face difficulties and may not win : they would be replaced, which would mean new faces and ideas at future EU meetings !
Should Theresa May action Section 50 in March 2017 as she said she would, or should she wait until after the result of the German General Election in October 2017 ?
In the meantime more changes will have been introduced in the EU. This will then, really permit Britain to decide if, it really does want, to be part of the NEW EU !