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http://www.legislation.gov.uk/ukpga/Vict/24-25/100/section/35
The 'furious' thing comes from an old piece of legislation originally intended to punish irresponsible driving of horse drawn carriages where injury resulted.
Its applicability comes from an 1879 case Taylor v Goodwin which held that a bicycle was a 'carriage', leading to another bit of legislation in the Local Government Act 1888. This is all a bit dull but shows just how muddy the law is in this area.
According to an article in the Guardian, bicycles ARE exempt from the speed limits as they're not mechanized vehicles.
But..
There was one guy done for "furious cycling"
and Richmond park and other Royal Parks are on different rules.