Rulebook actually states cage must be certified at 9.99 or quicker. Cage required at 10.99 unless all the original floor and firewall is in place excluding wheel tubs.
Here is the actual verbage and the answer to the original question is 7.49

4.5 ROLL BAR

Roll bar mandatory in all cars running 11.00 (*7.00) to 11.99 (*7.49) and in convertibles running 11.00 (*7.00) to 13.99 (*8.59), and in dune-buggy type vehicles running 12.00 (*7.50) seconds and slower. Permitted in all cars. See General Regulations 4.10, 10.6.

4.6 ROLL CAGE

Roll cage mandatory in cars running 10.99 (*6.99) or quicker, or any car exceeding 135 mph (217.2 km/h). Full bodied cars, with unaltered firewall, floor and body (from firewall rearward, wheeltubs permitted), running between 10.00 (*6.40) and 10.99 (*6.99) roll bar permitted in place of roll cage. Cars running between 7.50 (*4.50) and 9.99 (*6.39) must have chassis inspected every three-years by the ASN and have a serialised sticker affixed to cage before participation. Must conform to specs for body style used, as listed in General Regulations 4.11, 10.6.