Gah. That's a big fine.
Thing is, you booked a seat and paid to be on a particular train. There may well have been someone standing and not able to sit down because of that reserved ticket sticking out of the top of that seat. Indeed, if everyone who had booked to be on a particular train got the wrong one, there would be people unable to sit down, or people who have paid lots to be on any train unable to get on.
From their point of view, they have probably given you a heavily discounted ticket. You do not say how much you originally paid for your return journey - but I'm guessing it wasn't £119.50 but some kind of off peak super saver discount jobby?
If you then just get on any train, you are "taking" the rights of someone who had paid full fare, but without paying as much, which isn't really "fair". If they didn't fine like this, people could buy the cheapest fare, get any train they liked, and be quids in. If they charged a "nominal" fine amount like say £30, same as a parking ticket, once again you'd still be quids in if you'd only paid £40 for your original ticket, so people would purposefully do this to save money - it'd be on Moneysavingexpert.com for sure!
Rail travel is frightfully expensive in this country and I'm not really defending the train operating companies in that it is disgraceful that people who have paid for tickets dont have seats etc but to be fair that is a separate issue and one that isn't really going to be helped by people getting on a train they are not booked to be on.
If its any consolation, most of them are not making much profit and the running of the trains is generally one big headache.
Their customer service is generally shite though, it has to be said.