Whether the ticket (forget about itinerary, booking, PNR and other made up terms) is "overly ambitious," it is within MCT and EC 261/2004 does not look at "overly ambitious." CX sets the MCT and if CX wants to avoid the risk on flights departing the EU it could set the MCT higher. But, it has not. Perhaps it will.

To those who suggest that the delay is measured at the connection point not the final ticketed destination, the issue is currently before the CJEU and will be decided soon enough.

OP would be a fool not to make a claim and pursue his claim through the MCOL / Small Claims process in the UK unless he is willing to let EUR 600 go. If the matter goes his way, CX will pay up and if it does not, he will lose and will have wasted about an hour more of his time.

Perhaps the Regulation ought not to have been passed and perhaps it ought not to reward the "overly ambitious" but it does. So long as it does, why shouldn't OP at least try?