Diving with open Airbrakes....

[Lightning Mate]

"Agree, the Aircrew Manual says 650/1.3 for the F3 and F6 and 650/1.2 for the Tub"

If I remember (and I flew the 3, 5, and 6), 650 kts was the IAS limit.
In terms of dynamic pressure, therefore, there was no IAS limit on the use of the brakes.