As far as the carrying of knives is concerned, travel by rail in the UK is not unlike travel by air.
You won't be asked to walk through a metal detector on every train journey, but you can be asked. If you refuse, you will be denied travel.
Many people misunderstand the nature of our railway system, and comments like "it's a public place, so [random faulty deduction]" are a common result.
Between 1994 and 1996 the government sold all our railway assets to private organisations, many of which were set up expressly for the purpose.
http://en.wikipedia.org/wiki/Privati...f_British_Rail
As a result, all railway property in the UK is private property, not public property, and the property owners are entitled, within reason, to forbid whatever they want to forbid.
Given the large amount of criminal activity which takes place on railway property, some of which involves the use of knives, the rail operators have taken a fairly strong stance on carrying knives on railway property. They have simply forbidden it. They will occasionally ask passengers to pass through metal detectors in 'intelligence led' operations designed to deter the carrying of knives and other items which could be misued. If you get picked up by one of these operations, expect at least to spend some time trying to explain why you were breaking the rules, and prepare for the possibility of your travel plans being modified by your arrest. Don't expect to see your sharps again unless you're lucky enough to have met an especially understanding member of the British Transport Police who happens already to be over his performance criterion.
http://www.btp.police.uk/passengers/...ife_crime.aspx