If you like to see museums, musicals or operas, stay one week in Landon is enough. If you also want to see sights around Landon, probably you need two weeks.
Lake District in the North Western England is also good for honeymoon and summer vocation. You can visit Peter Rabbit’s home.
The last suggestion is go to Scotland. It is very good season to visit Scotland as it is not cold in summer. And it is on sale season. You can visit Edinburgh Festival in August. Tattoo is really worth to see. You also can go to Invernes and its surrounding area like Loch (lake) Ness with its famous Monster Nessie. Most B&Bs are by the river in the Inveness. It is such romantic that very suitable for honeymoon in my opinion. You can spend about one week in the North Scholand.