Easyhotel is a very budget-oriented hotel but one that I do find myself coming back to. They've got a good dozen with most based in city centres all over the UK, so naturally close to train stations. If I'm in London for the night I'll pretty much always stay at the Heathrow one, which is pretty close to Hayes and Harlington station.
If I'm in Edinburgh, I'll always stay in CODE Pod hostel. It's a pod-based system so you get privacy compared to other hostels, and it's always remarkably clean. Plus you get a free breakfast too IIRC.