The only clear requirement to limit deflection of glass in British Standards is given in BS 6180, which limits the displacement of glass in barriers to 25mm. This displacement would include any movement of the support to the glass.
BS 6262 has design curves which appear to be based on stress limits and calculations suggest that storey height panes of toughened glass complying with BS 6262 could deflect by 40mm under full wind load.
BS 6262 contains a note to warn that glass having to withstand only low wind loads or glass used internally, might require to be increased in thickness to keep deflection within acceptable limits. No guidance is given on how this should be achieved.
Providing the glass is within allowable stress limits, the issue is not of safety, rather one of perception. If building users see glass undergoing fairly large displacements they perceive this to be unsafe and it causes concern.
This has proven to be a particularly contentious issue. We are aware of situations where glass used has been selected in accordance with the guidance given in BS 6262 which has subsequently been rejected by the client due to in their eyes excessive deflection. We would therefore encourage anyone with experience of this to get in contact with us.