Glad that you are making your first trip to England! I hope you have a wonderful time!
I doubt you will be asked about U.S. politics while you are in England. None of the very nice people we met when we were there this May asked about this. If you are wanting to take a break from all the political rhetoric on U.S. television right now, don't talk about it while you are in England if you don't want to. Start conversations instead about what local pubs, restaurants, or historic sites your B&B owners recommend, for example.
You asked about talking with people on a train or in a pub.
We were recently in England for three weeks (May 2016) and no one engaged us in conversation in a pub or on a train. We would have engaged them in a conversation, but the opportunity did not come up.
"What conversations are you having and where? What should we anticipate?"
During our May trip, we found that those people who were most open to having a conversation were:
1. B&B owners or managers.
2. Cab drivers in London.
No one asked us about U.S. politics while we were there. The most chatty person in London was a cab driver. This was because we had booked him for a long cab ride, and he seemed to want to pass the time in conversation. He opened the conversation by asking where we were from in the U.S. When we said "Nashville", he started talking about country music, and Elvis! Then we asked him how many years he had been a cabbie. He said "40 years", which led to a lot of interesting stories about the passengers he's carried! Hugh Grant, Judy Dench, and Patricia Routledge, to name a few. The latter two delivered to the theatre where they were to perform that night.
3. Guides, docents, or staff in tourist sights. For example, the Beefeaters at the Tower of London will be glad to have a conversation with you. Thankfully, it will be about English history--a subject far more interesting than current American politics.
I agree with Rick Steves' philosophy that it is interesting to talk with people in other countries and to learn about their cultures, politics, and opinions. But if that discussion may cause hard feelings, or is uncomfortable for you, don't talk about politics. You will find that people in England love to talk about their town, their favorite pub, nearby historic sites, or other places in England you should stop and see on your trip. These are good talking points, not controversial.
Have a great trip!!