I'm a train guy so I enjoyed my two trips on British trains (one to York, one to Oxford). A few things I noted:
As Rick notes, London has a LOT of train stations - I thought Paris took that prize but no. So make doubly sure you're looking at the right station and then going to the right station. Sometimes as with St. Pancras and Kings Cross they are across the street from each other which is pretty funny, so again pay attention.
Speaking of which, for some reason the station won't tell you what platform your train is at until about 10 minutes before departure. So there's always a huge scrum in front of the info screens of people waiting and waiting and waiting and then, when it's finally posted, they all rush off in the same direction. Which you should do as well, since the trains are quite long and if you car is up front, after you negotiate the turnstiles there's a long walk ahead of you. Your ticket should indicate which car you're in and there are maps of the train layout as you enter the platform. The turnstiles are a bit tricky since they have to accept several kinds of media, just take a breath and figure out what you need to do to get past.
There are several rail companies in the UK but seem to share the same tickets. Which is odd but helpful, my desired return from Oxford to Paddington was cancelled but there was a train to Marylebone about to leave so I asked the guy at the info desk and he said sure, your ticket is just fine. And expect delays, my train TO Oxford was delayed waiting for a worker and then at Reading we stopped and waited again for ANOTHER worker to show up.
Speaking of tickets, on my four train rides I was approached by no conductors. The only workers I saw on the train were collecting trash from passengers. I guess the companies have decided that if you can get past the turnstile you belong on the train, or something. Or possible losses are less than paying a body. There are constant announcements saying if you see something say something, but there's no one to tell. A group of drunken louts got on one of my trains and serenaded us with songs and bawdy jokes and there was no one around to tell them to shut the hell up; they got off after one stop and I wouldn't be surprised if they didn't pay.
The ride is pretty comfy and there will likely be wifi and power sockets (remember to bring those adapters!). Announcements are frequent and there are scrolling signs of upcoming stops. If you think you'll be hungry or thirsty it's best to stock up at the station beforehand and then eat at your seat.