Depends on what you like to do. I know London fairly well I guess.
London is full of parks from formal gardens, to overgrown churchyards and one of the jewels is Richmond Park and another (closer to the centre) is Hyde Park.
What are you looking to do?
sorry im meaning more national park/landscape/scenic regions surrounding London - reasonable to travel to from there
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)