Notwithstanding the TAFs and what may have been happening elsewhere in the UK, the METARS show that at Shoreham (20 miles west of Beachy Head) the surface winds were light easterly all afternoon. They. had been light south easterly in the morning. If this was representative of conditions at Beachy Head then the chances of successfully soaring the cliffs would have been minimal.
I suspect that if the crew of the glider had been aware of this they would not even have bothered to take off. They would have needed to motor much of the way to and from Beachy Head.