Generally if the aircraft is delayed, and is advertised as such, then conformance moves with it, so long as the delay isn't a few minutes. However if they find another aircraft at a late stage and swap it over, you may find yourself leaving close to your original time. Also if there is creeping delay then that doesn't work so well, not every 10 minute pushback gets reflected in conformance. But let's say the service is showing as an hour late, you get to T5 security an hour before the new departure time, then you would be let in, worst case scenario is you will have to visit the Visa Check desk nearby to unblock Ready to Fly.