Those tend to be a little further outside London I think.
You have Peak District, Lake District, Brecon Beacons, all a few hours / train ride outside London. More traditional areas closer to London like The Cotswolds.
All of Scotland is amazing though!
The south coast of England - South Downs - has some beautiful parts although I don’t know it well.
Be aware long distance train tickets are quite expensive and fuel prices also are quite high at the moment (although you wouldn’t need a car in central London)