In Germany I usually buy my ticket on my phone on the way to the station (I do have a BahnCard 50, i.e. 50% off all journeys). I could typically save a few quid

In Germany I usually buy my ticket on my phone on the way to the station (I do have a BahnCard 50, i.e. 50% off all journeys). I could typically save a few quid if I booked non-flexible tickets a few days in advance (3-7 days). For longer distances (>400km) the savings become more noticeable and my trips less spontaneous, so I do tend to book those in advance as far as possible. In the UK (I had a Railcard), I booked everything as far as possible in advance (quite often on the day advance tickets became available), as prices for advance tickets do vary quite a lot when you get close (weeks instead of months) to the day of travel. If you don't care which train you take, then you'll usually find affordable tickets a week in advance, but when meeting people at a fixed time, booking close to the day can yield unwelcome surprises.