Ooh.. what a fun topic.
Whereabouts does your LDL live? I'm asking because you mentioned you hadn't been to London sightseeing even though you'd been to England on 3 separate occasions. I figured maybe he lives quite far
I also didn't get to visit London on my first trip (2 weeks). It was only when I went there the second time (for 6 weeks) that we got to spend a week in London. It was great fun, but not sure I'd want to live there.
What you'll enjoy there probably depends on what interests you. There are tons of amazing guides for in and around London. There are the top attractions of course, but also many many hidden treasures.
In my own experience visiting there, I found that I enjoyed wandering the streets more than most of the actual touristy things. I liked going for a pint and having a stroll around more than anything really.
One of the top tourist attractions that I really did enjoy was visiting Buckingham Palace -- but not during the day for Changing of the Guard with 10,000 other tourists. No.. we walked from our nearby hotel to the Palace around 10:00 PM (during the summer months so there was still a bit of light in the sky). It was amazing getting to doddle around London as dawn set in. When we arrived at the Palace itself, we found we were literally the only people there. The whole place was lit up as was the waterfall that stands proudly out in front. It was amazing getting to walk right up to the gates without having to struggle with crowds. It was an odd sight to see such a famous landmark virtually devoid of anyone but ourselves.
From there, we walked to another nearby castle/palace (forgive me, can't recall which one, but the nearest one just down the street) and had a good long friendly chat with one of the guards there.
Was a beautiful night and definitely worth it. We returned the next afternoon so I could watch the Changing of the Guards. Unfortunately, every other tourist in London did just the same. All I really saw was the back of many heads while I stood out in the blazing sun squinting my eyes. Those were 2 hours I'll never get back. :-\
The London Eye is also quite enjoyable (even though it broke down while we were on it). Good views and gives you an accurate layout of the city.
Another of my favorites while there was getting to watch the FIFA World Cup with thousands of mad fans both in Trafalgar Square (Brazil vs. England) and in a sports bar in the centre of London (Italy vs. S. Korea and US vs. Germany). We were cheering on S. Korea and were elated when they won.
There must've been 3,000 S. Koreans and the pair of us in that bar when we all spilled out onto the streets cheering and chanting.
Anyway, think I'm rambling on here. I'm sure there are people on this board who live there who would be more of a help than I could.
I think it's fun to see the top sites that you know you're likely to enjoy, but I wouldn't devote too much time in museums unless you're incredibly eager towards that sort of thing. It's never what you remember about a visit.
BTW, a book I found to be of considerable help when deciding what I thought I might like to see and do, and stay and sleep and shop and all of those other good things.. is one of the "For Dummies" Series, appropriately titled, "London for dummies."
There will most likely be sections of the book you can skip over, such as information about obtaining a passport and rules of travel and such. But I do recall a very good couple of maps as well as suggested outings for every taste.
Hope this helps some!