purely as a data point (I would also build in a night-stop or a long buffer), BA828 arrival this morning, it took 15 mins do passport control and security and be back airside. no queue whatsoever at Fast Track security (though there was a queue at normal) at 10.30am this morning. that's just today. I had built in a 6 hour buffer (and moved to that from an overnight stop as I had a bad feeling about the last flight of the day - which got cancelled 2 days after I moved myself).