As far as I'm aware there are no constitutional laws about such things as Referenda, no doubt because they are such a rarity.
The Referendum that took us into Europe in the first place was considered to be Mandatory.
Take a look at https://en.wikipedia.org/wiki/Referendums_in_the_United_Kingdom
Then "be aware".
The referendum in 1975 occurred 2 years after joining the EC, it was also a "stay/leave" vote. At that time the full implications of membership were understood, and leaving would have been less of a "leap in the dark" than now as it was a short while since the previous situation and the setup was clearer. So the vote was well defined with clear consequences, unlike the present where staying is well defined but the leaving is just a chaotic mess. And it WAS NOT mandatory.
I suspect that those MPs wanting to delay or amend the current action are fully aware that there is a small majority in favour of "leave", but are also aware that an even greater majority would be angry and upset with any governing body that just drove off a cliff. Either they are highly principled and honest, or "self serving" in that they feel the potential damage of the fall-out is a greater blow to their career then that of delaying or re-negotiating. Same effect either way.