popay - the use of discretion - if required - is considered to be a 'normal part of an FDP' by the CAA, as B says. An airline cannot re-roster a flight because the NAT, wind or bad slot makes it creep over FDP, but relies on the Captain to judge and use his 'discretion' where applicable.
The authority will sit up and take note if excessive discretion is used on a regular basis on a route and have acted in the past to stop this. If you feel the trip is regularly running into discretion, do as B says. What is not acceptable is airlines who regularly plan within 5 minutes of FDP or know that the planned t/round cannot be achieved. I believe the CAP guideline is that trips should be planned to FDP limit at chocks+30 - and this normally copes.