Writing to the local paper is probably a waste of time (they mainly exist to sell papers not obtain financial redress for such things) - but I would consider enlisting my MP in a letter of complaint to Avanti west coast seeking a goodwill payment for what was clearly an honest mistake made by the OP
OP - the policy is designed, I assume, to prevent passengers using the very fast trains run by Avanti from filling seats with short distance commuters going to and from London to Watford so they commonly have a restriction such as 'stops to pick up only' going north and 'stops to set down only' going south (so that a person travelling from say Watford to Manchester can make that trip without having to go 1st south to Euston before heading north to Manchester). I believe the same applies to certain other equivalent stations a relatively short distance form London on other routes (if not I think it once did anyway)
Not that you would have had to consult one but this is usually made clear with a symbol and footnote in the paper (now pdf) timetable
as shown here -
see the 'u' or the 's' next to the time of trains stopping at Watford Jct
and see the key for example on page 38
Still, you have my sympathy - I would have hoped that you would have got ticked off by the staff and that would have been the end of it - but I don't know what instructions management will have given them on how to deal with such matters. Plus I suspect that due to this policy avanti receive no money from the ticket you had bought since they do not carry passengers from Watford Jct to Euston, which may be part of their reasoning if such charges are their course of action.
I would certainly write to them and ask for a gesture of good will, if that fails, write to your MP and ask them to raise it and see what happens. Worth a try / cost of a stamp / e-mail IMHO.
PS
What he meant by that was that you were supposed to wait on the platform until the second train arrived and get on that, as I assume that would be a train run by London North Western Railway on which the ticket you had would have been valid to use to get to Euston.