GWR
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Under an hour on the fastest trains, with Christ Church, the Bodleian and a river punt all within walking distance of the station. Oxford is one of the easiest first day trips out of London — no bus connection to plan, no missing station, just a straightforward train there and back.
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A direct train from Paddington, under an hour and a half, into a city small enough to see properly in a day. Bath is one of the few places on this list where the day trip logic actually works without qualification — no bus connection, no missing station, just a straightforward train there and back.
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Under two hours from Paddington to the Welsh capital, on trains that run twice an hour on weekdays. Book the right fare type and it’s one of the cheapest long-distance journeys in Britain; book the wrong one and you’ll pay three or four times over for the same seat.