Windsor castle

If you've got at least 6 hrs you can see a bit of London; 5 hrs for Windsor. With the Apts & Chapel closed, the tour may not be worth it, but, if you are able to be at the castle at 11am, you can see the Changing of the Guard & then explore the town a little.
Due to the volume of people passing through security at LHR, it's a good idea to be there 3 hrs before your next flight.

Can you specify how long your layover is. As previously mentioned you would need at least eight hours for London (hour to get out of airport, hour into London, two or more hours to look around, hour to get back, and three hours to check in) and maybe six hours for Windsor. If you got the time, I think it's pretty much always worthwhile to get out of the airport, especially if your destination is close. Windsor is close enough to make it worthwhile.

The State Apartments and St. George's Chapel will be closed so they can get ready for the Garter Ceremony on Monday the 13th. The Trooping of the Colour is in London Saturday the 11th...so London will be very crowded. The Queen and other members of the Royal will be going out the Windsor Castle later in the day. Not knowing how long your lay over, but if you would be in Windsor early evening you might be able to see the Royal family coming into Windsor Castle. The Royal Family will be staying at the castle that week because the Royal Ascot starts Tuesday the 14th. The Royal Family uses the 3 mile Long Walk which is the private road for the Royal Family, in Windsor Great Park. You get a great view of the Royal residence at the gates in the park. I think Windsor would be your best option to visit on your layover. You could walk to Eton (8 minute walk) and see Eton College where the Royals and upper class send their sons to middle/high school. There are great pubs you could get a nice meal. I think you would enjoy your time there. Transportation to Windsor is easy.....a 15 minute taxi ride.

How long is your layover. Probably count on at least half hour to an hour to get through immigration and customs (depends on how many flights are in at the same time) and allow three hours to get back in though security- it can be a bear. Travel to and from London. how much time is left over? If you are young and fit you can do a good interesting walk along the south bank from westminster to the Tower. (Trooping of Colours clogs up the Horse guards, Mall and buckingham palace area). An outside walk through Windsor Castle could be interesting plus a stroll through the town. Unfortunately the Chapel is a highlight and you wont see that. Difficulty is that Windsor is not on a Tube line so you need alternate transportation to/from Heathrow