If you lose your tickets, you have not committed a recordable criminal offence under the Regulation of Railways Act, but if you are asked to present your ticket, you have committed an offence under the Railway Byelaws, albeit a non-recordable offence, for which you can be prosecuted.
If you lose your ticket before boarding the train, you have committed a byelaw offence, regardless if anyone asks to see it.
In any area not designated as a compulsory ticket area, no person shall enter
any train for the purpose of travelling on the railway unless he has with him a
valid ticket entitling him to travel.
Quite how you would be found guilty unless your concious causes you to "fess up" is anyone's guess, but technically I would interpret this that you would have committed an offence, just like if you do 75mph on the motorway and noone catches you, you've committed an offence.
If you lose the ticket *on the train* though, you haven't, as you'll have already entered the train.
There used to be bins on exits of platforms and/or stations for used tickets. I'm not very observant, do they still exist?