It's been illegal since 1835 (Section 72 of the Highway Act) and it remains illegal.
Under the Fixed Penalty Offences Order 1999 (combined with the provisions of the Road Traffic Offenders Act 1988), 'cycling on the footway' can now be dealt with by a fixed penalty notice.
Our local community constable has a monthly column in the free newsletter that is delivered to every house in the area. He regularly reports handing out penalty notices but the number seems to be falling as local cyclists 'get the message'. Other police forces, including the Metropolitan Police, have been 'clamping down' on this offence since the fixed penalty measures were put into place.
Chris