Apologies,I'm not sure what you intend to mean by one braking system.
He didn't comply with 7i, and if you read fully into the law, he didn't have one braking system. Having a fixie is a legal excuse to have only one braking system. Without it he had none.
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I have re read the section again and find I must agree with you in part.
The fixie constitutes a braking system, so he can claim to have had a brake.
However the same section requires that where the wheel cannot move independent of the pedals, ie a fixie, that it must be the front wheel, otherwise there must be a front wheel brake. So he will be done for not having fitted a front wheel brake.
However,
I cannot see a manslaughter conviction being returned or surviving an appeal.