On what basis is it a criminal offence to include a disclaimer hellywobs? Where did you get that idea from - a bloke in the pub? It's not illegal at all (or else UKA and the FRA amongst others wouldn't require things like this!).
However, as has been stated, an organiser can't exclude liability for anything caused by the negligence of the organisers or their staff, but they can exclude liability for the stupidity of many runners (and believe me, many are stupid in different ways on race days - I've seen it all over the years) who act outside of what a court would deem to be reasonable. Conduct has to be "reasonable" on both sides of the fence - competitors have a duty of care to the organisers, other runners, spectators and road users - a lot of runners seem to forget this responsibility when they put their running shoes and a race number on!