My understanding (may be wrong) is that Julian Amery (a British politician) had the 'interlocking' clause written into the Anglo-French contract because the Brits thought the French might want to pull out. Harold Wilson did want to pull ou, but couldn't because of that clause. Were the French ever prevented from pulling out by the clause?
If both parties had wanted to pull out, presumably they could have re-negotiated the contract to allow that.