Our family of four (including 2 boys, ages 8 and 15) took a 10-day trip to the UK in mid-March 2012. Time was split pretty evenly between London and visiting family in Leicestershire, Nottinghamshire, and at a family reunion at a youth hostel in Peak National Park.
We flew British Airways between Newark and Heathrow, and the flight was fine- the seatback entertainment kept the boys quiet, and there was just enough legroom (I am 6-6) to manage. Customs was a breeze- quite a contrast with the return to Newark with the $5 for luggage carts, long walks down passages that still look they were a temporary setup cobbled together yesterday rather than a careful permanent layout, etc.
London was terrific. We visited the Churchill War Rooms, Tower of London, The London Eye, London Dungeon (the boys loved this), the Imperial War Museum, and took a boat ride down the Thames to Greenwich.
I highly recommend the Churchill War Rooms- the Churchill museum in the middle of the tour is just astounding. I also highly recommend the boat ride down the Thames, if the weather is inviting.
If you are a Doctor Who fan, I also recommend a visit to the Who Shop not far from the Upton Park tube station- the most doctor Who merchandise I've ever seen under one roof- they also have a small museum with props from several of the stories.
We stayed in a flat in London, and had Travelcards to get around on the Underground and buses. We actually went back to Heathrow to pick up our rental car and head north- we figured it avoided driving in central London, and made sense since we would have to end up at Heathrow for our flight back anyway.
We also visited the National Space Centre outside Leicester, and that was surprisingly good- I wasn't aware that the university apparently has all sorts of partnerships with NASA and the European Space Agency. Would have liked to spend more time here.
The countryside in the National Park was simply stunning, and the weather was just about perfect. apparently there are some caves worth exploring just outsidethe National Park, near Buxton, but we got sidetracked by a used bookstore- we ended up bringing home so many books we had to redistribute them among everyone's baggage to stay under weight allowances.
All in all, a great trip- I saw some family members at our reunion that I had not seen for over 40 years. Hope to do it again some time if we are ever solvent once more- we had to refuel the rental car before taking it back, and while the car, a diesel, got great mileage, I still paid over $100 to fill the tank- I don't know how people do it if they are outside London.
I'll also have a couple of extra postings on some side issues that I was interested in, and for which our experiences may be helpful for someone else.