I also vote for just buying the train tickets there. Your schedule is more definite the closer you get to the trip, and you can clearly see the options.

The big machines have English (hit that British flag) ball. They take various credit cards in addition to cash. The windows sometimes aren't staffed with English speakers, but often are.

You don't have to carry those train tickets on your overseas flight there, and then pack them around for the days until your rail trip. Less chance of losing them too.

I've always been suspicious that there were more schedules offered right in the train station than there were on places like raileurope, but don't have any evidence to support that.

Romelle