I'm surprised no one (including TFA) has mentioned Trainline yet, as it addresses a lot of the issues mentioned in the comments here:
- buy tickets (with easy explanations of when/where they're valid)
- see which platform a train will depart from/arrive at
- check delays
- see walking times between platforms
- bus connections info
- handles refunds
- etc.
It works across Europe (elsewhere, I'm not sure), but in my experience it's got the most functionality within the UK.
The app itself is fantastic too. I'm not exaggerating when I say it is one of my favourite pieces of software - it does 1 thing, and does it super well. It's fast: no fluff, no ads. Few apps on my phone are like that anymore.