England in 2-3 days
If you've only got a few days in England there is no point leaving London.
The capital has more than enough attractions to keep you moving and you'll wish you had longer just to explore it.
In 7-9 days
Spend 3-4 days in London before heading down to Brighton for a night (or just for some fish and chips on the pier).
After a walk along the pebble beach head inland to Salisbury and explore the small town or head straight to the nearby Stonehenge. Wander around the mysterious stones then continue on to Bath for an overnight stay.
Take a tour around the Roman Baths and the Abbey. After leaving Bath you could head east through the Cotswolds towards the university city of Oxford or north to Stratford upon Avon, the birthplace of Shakespeare.
Continue north to the Lake District before heading back to London.
Two weeks +
With more than two weeks to travel around England you should consider spending more time in London, plus checking out some of the other major cities such as Manchester, Newcastle-upon-Tyne or Liverpool.
Also worth a look is the walled city of York (plus the nearby North York Moors Oxford), the Cotswolds, Cornwall and Canterbury.
Plus you shouldn't leave without visiting Scotland and Wales along the way.