All circuits have noise limits and Brands Hatch is no exception.  Given the circuits proximity its noise restrictions are lower than most other places and inforced quite strictly.

The Indie circuit has a 105db limit;
The GP circuit has just a paltry 101db limit.

The 101db limit is relatively easy to get through.  However if you have modified your exhaust you may run into trouble. You can wiggle through the sound test by not reving the car as high as it should go.  The regulations say rev your car to 75% of the redline, so a little lower and you may get away with it.  The problem is that there are db metres on track and if your car trips one you may get sent home.

You can buy aftermarket baffles that attach to the end of the exhaust pipe.  They will help get your noise levels under the limit but will also reduce your power output, but not too much.