24 hour layover in London

Yes Wedding Day is a national Holiday. I would expect that changes Tube, bus and train schedules. Will also make hotels more expensive and difficult to get (espeically for one night right before the wedding).
Its going to make things difficult for you

You'll leave Heathrow around 3 pm on the 28th and need to be back to check in around 9 or 9:30 the next morning (really). That doesn't leave you with an excess of free time. I'd book a hotel at Heathrow. There are one or two very nice and very expensive hotels actually at the airport, and a number of less expensive hotels around the airport perimeter. I'd book with one of the latter. Check in, leave your bags, go into London, come back late at night, get up and catch your flight. Things to remember: No hotel in London that you'd really want to sleep in is cheap. The Tube run from Heathrow into London is on the Piccadilly Line. Takes at least an hour. Lots of stops. And, it's a subway. The Heathrow Connect is a train that make a few stops and takes 30 minutes or so to get into London. The Heathrow Express is a comfortable train that takes 15 minutes of so to get into London. So... I'd trade a few dollars for time. I'd take the Express into London, see as much of London as I could in that short time, take the Express back to Heathrow, catch a shuttle or a cab back to my hotel, get that extra hour's sleep because you're at the airport, and fly out. FYI: You obviously won't see all of London. Without knowing your interests, I'll suggest getting to Trafalgar Square by cab, walking down through Whitehall to Parliament and Westminster Abbey, walking across Westminster Bridge and turning left to amble along the South Bank. Grab dinner at a pub or a cafe along the way. Take the Tube back to Paddington Station, and the Express from there to Heathrow. Note: The Tube and trains do not run all night.

An alternative to going into London is to visit Windsor- either overnight or do as JC suggessted and sleep at Heathrow. Arrange a pre-paid car service to pick you up at the airport as the black cabs waiting out front will cost much, much more. Lots of things to see and do. It is a lovely place to wander around. Eat there before heading back if you sleep at a Heathrow hotel.

I have been to London several times. My last layover there I went to Windsor, which I really liked. Any other suggestions as an alternative to central London or Windsor? I keep looking at books, maps and websites for some ideas, but nothing beats info from fellow travellers!

Check into a hotel at Heathrow, go into London as soon as you can for a little sightseeing and possibly dinner and theater. Back out to Heathrow for the night. No worries about getting to the airport on time.

j.c.'s advice is good, but may be difficult that day. Because the Abbey is where the Wedding will be the next day. I would expect a very high security presence and probably pitches for rough sleeping, so busy and crowded. Anything along the route of travel for the Couple will be similar.

I hadn't thought about that, Nigel, so thank you. If it was me, I'd hide out in the British Museum all day. But, I doubt that's what Jean has in mind. I assume it's safe to say that not only will the wedding impact Tube schedules, but that it will also increase ridership and make cabs harder to find.

I second j.c.'s suggestion to book a hotel in the airport area, and do it right away, or you will not get a place to stay. People were posting here a month ago, saying they couldn't find a room anywhere near London or in London the week of the wedding!! Your main concern should be not missing your flight out the day you leave, so you'll want to be close to Heathrow. As for going in to London for a walk around town the day before the wedding, just know that all the attractions, pubs, and cafes will be jam-packed with people. I doubt you will be able to get anything to eat or drink without a long wait, if at all. Still, it is worth it to walk the route j.c. suggested. Walking all over town and looking at the landmarks is free, of course, great for the budget. Also, if you can get into St. Paul's and walk all the way to the top for a view out over the city, that is something one never tires of. Even if you've done it before, still breathtaking. If you choose not to go into the city the first day, may I suggest a trip to Hampton Court, unless you've already seen it. The day you are supposed to leave, I suggest: don't do anything except get to your flight ahead of time to check in. Don't go into the city that morning; you could run into a delay returning to the airport to catch your flight out. There could be all kinds of traffic and transportation snafus due to the wedding and crowds.

Jean, We are traveling to England next month for a different wedding. Tho we already have our own reservation in town, it was suggested to me to look into the Master Robert Hotel. It's in Hounslow so you're near the airport. I think one of the "remarks" said it was a bit of a walk to the tube station but you might think it's worth looking into. If I only had a part of one day to spend in London and I'd spent a lot of time in London already, I'd go to the V&A. And yes, I have taken the Milk Run from the airport. Very interesting and we will prolly go that route after "our" wedding in Cornwall, when we then spend a few days in London.

Thanks Laurel (& everyone)! The V&A is exactly the place I am thinking of going. I haven't been to the floor with all the clothes. They close at 6:45, so if Ican get there in a reasonable amount of time, that's what I'll do. I booked a room through B&B Net today nearish the Osterley tube station. I have never booked through and agency, so I hope they are reliable. I'll post a recommendation if it works out! Thanks for all the great advice!

Jean
After you visit V&A Museum, think about a walk through Kensington palace gardens and Hyde park. The days are long at that time of year, and if its not raining it will be very refreshing- besides you will want to stretch your legs before your flight home.

Here's an alternative to London on wedding day: Go to Oxford. It's about 90 minutes away on the buses that run between the airport and the city. Many trips per day, and they run until near midnight. That would open the opportunity to stay the night in Oxford, if you are willing to catch an early morning bus back to Heathrow. Google "Oxford-Heathrow bus" and click on the first result.