I'm traveling to London, Paris and Italy in two weeks. I've only been to Italy in the past and am well aware that you have to validate your train tickets. I was wondering if this was also a fact in London and Paris.
It is certainly a requirement if you are using a rail pass. Is that what you meant?
British train tickets don't need validating as they have their dates of validity printed on the ticket.
I'm not sure about Paris or France.
If you are taking the Eurostar (chunnel), boarding is handled about the same as an airplane, so validation is done by the attendant. The only tickets I had in France were either a railpass (validated at the start) or a ticket for a specific train and time that I do not recall validating.
You need to validate in France.
Unfortunately, it's so easy rush onto a train and forget to validate your ticket!
As Sharon noted, train tickets need to be validated in France. The validation machines look very much like the Italian ones, except that the French machines are orange.