Cameron regrets allowing the British people the opportunity to express themselves, finally. Now wants a second referendum.
What's the point in referendums if they are called again unless they give the answer you want?
The problem isn't the first referendum. It's the parliament which refuses to honour it. It's the conservative MPs who voted Remain but pretend they are concerned about a No Deal exit, leaving Brexit in limbo.
Let democracy express itself by giving people the opportunity to kick out these MPs and replace them with new ones who will honour the result of the referendum.
The only constitutional issue is MPs who refuse to honour the lawfully-expressed will of the people.