Oi, shouldn't you be using
www.biketrackdays.co.uk to look up your prospective track days? Much better sit ebut I am a bit biased.
Don't worry too much about the actual dB limit. If you go to Castle Combe you will need the standard can.
If you go to Donington you will need to check whether it is a quiet or noisy day. On a quiet day you will need the standard can.
Last year Brands GP dates were either noisy or quiet although none of the organisers actually told anyone!
Most of the MSV circuits will be enforcing their noise policy but in almost all instances a race can should pass the noise tests.