Basically all the major cities in England and Wales are perfectly doable as a day trip from London by train. Newcastle and Swansea are about the limits there at ~3h each way, but most are significantly less than that. We're really quite a small country, at least compared to the US!

If you want to travel early on a weekday, day trips can be very expensive as standard peak fares are extortionate. But buying in advance you can get very good prices indeed, or if departing after morning peak then off-peak day returns can be good value depending on route.