TrainPal - The best. No fees. Split tickets save a lot. Accurate and timely travel updates tracking your train, notifying you when you're about to arrive. Many discount codes and you can store them in app. Railcard can be stored in the app.
If you need to print split tickets, you might need a customer service desk. Ignore the "spin to win" thing.
Up to 10% off codes: RG04, TCB20, TPAL25
Uber trains - Gives 3% cashback (10% with Uber One), but no split tickets, so likely to be more expensive. A few minutes delay between buying tickets and getting them emailed to you. Sometimes sold me invalid tickets, or couldn't complete the booking.
Trainline - Charges fees, offers terrible refunds instead of recommending Delay & Repay, sells digital tickets when some destinations need physical tickets still, cannot store your Railcard, RMT officials said don't use it.
Operator apps (GWR, Greater Anglia, etc) - no split tickets, no discounts, often sign you out, some will be defunct when they go back to public ownership.
For navigation:
tfl.gov.uk if you're going via London
nationalrail.co.uk for all else
Don't use Google Maps or Citymapper (inaccurate times, outdated info, no disruption info)
Other train hacks:
Barriers often open between 10pm - 11:59pm historically for the government-mandated "third class" slow service trains, so if you're doing a return trip late at night, contactless "where required" may be cheaper than a Travelcard for your full journey.
If you don't have a ticket and you're blocked by a barrier to exit a station, you "could" just buy an eticket from the next nearest station to where you're trying to exit.