London for 12 days in July....

Julie - The London Harry Potter tour would be ideal for your teens. I am sure they would enjoy that. In addition, day trips to nearby towns of Oxford and Stratford - upon - Avon (Birthplace of William Shakespeare) is highly recommended during your 12 days London adventure. Windsor castle is a must as well as the Sky Garden for a panoramic view of the city.

https://skygarden.london/

https://www.rct.uk/visit/windsor-castle

Some more information would be helpful. Older or younger teens?

I’d check out www.walks.com for a great selection of walking tours for £10 per adult. No need to book in advance, just show up.

We are going to London in April and I am researching our trip now. Besides London Walks which have some amazing walks, my husband is a big James Bond fan and the London Film Museum has an exhibition. Joolz Guides on YouTube has some fun videos of London.

On our last trip this past October, we went to a fabulous place, virtually “hidden” in plain sight, right downtown. Only a few other visitors were there, and I’d be surprised that it would be as busy as many other sights in London in July, but it was a surprisingly great part of our trip. We didn’t have teenagers with us, but I believe they’d find this of interest, too.

The Banqueting House, right on Whitehall street, south of Trafalgar Square and north of the Parliament building and Big Ben’s tower, is the one surviving building from the immense Whitehall Palace, begun by Henry VIII, expanded by Elizabeth I, and burned down years later. It’s also the site where, in the 1600’s, King Charles I was gruesomely executed by beheading, in front of a crowd of onlookers.

But before Charles’ execution, he had an amazing, giant painted ceiling put in the Banqueting House, where his throne sat, and where elaborate performances were staged. The hired artist was the famous, and expensive Rubens, who portrayed Charles’ father, James I, amid cherubs, wild animals, and scantily-clad women representing Wisdom and Justice. It’s like Britain’s Sistine Chapel ceiling, except the king was portrayed in place of God. There are bean bag chairs to lounge in while you admire the ceiling, easier on the neck than craning it up to admire the work, and very interesting audio guides, describing it all.

A short film downstairs, too, in what had been the king’s wine cellar. They also have period costumes, male and female, if people want to dress up and get a photo in front of the replica throne. The owners of our B&B, Londoners themselves, weren’t even aware of the Banqueting House, so it’s a treat that lots of people apparently miss!

There should be lots going on at the Underbelly Festival at Southbank Center on the south bank of the Thames. The 2020 lineup has not been announced, so check back closer to July.

http://www.underbelly.co.uk/what-we-do/our-festivals/underbelly-festival-southbank

Lots of entertainment, or just wander the grounds and have a bite to eat.

Definitely theatre. Have them review what’s on during their travel dates on the London Theatre website.

I can personally recommend Hamilton, Witness For the Prosecution, and The Play That Goes Wrong.

Climbing the O2 might be of interest if they are adventurous. The Tower Bridge Experience is interesting. Again if not fearful of heights see if they can climb the tower in Olympic Park.

There are speedboat experiences on the Thames. They can also kayak on the river.

London walks: Ghost tour. The Seven Deadly Sins.
Westminster Abbey. Jack the Ripper.

Imperial War Museum. Tower of London.
Museum of London. Museum of Water and Steam. Alexander Fleming Museum.

Walk, run, bike or fly a kite in Richmond Park.

Carnaby Street, Spittafields Market and Brick Lane Portobello Road Market, Mercado Mayfair, Camden Lock Market.

Check what will be on at Wilton Music Hall and Cadogan Hall.

As their travel dates draw closer check the TimeOut London and The Londonist websites.

Remember teenagers are allowed in pubs afternoons and early evenings. They don't have to drink. There’s food. Pub grub is perfectly fine. They’ll see mothers with babies in prams, grandparents, families and dogs in pubs as well. In July weather should be nice so any pub along the river will have an outside area to sit. Mayflower Pub, and the Prospect of Whitby are personal favs.

They’ll never be bored in London!

I love that you are having an extended stay! We’ve had 3 one-week stays, one two-week stay, and a few of fewer days. Always something to do!!

The London Transportation Museum at Covent Garden is fun and I second the recommendation for the James Bond in Motion exhibit. You’ll want to rewatch all the movies.

A day trip to Windsor is nice, if not thrilling, ditto Canterbury.

Make an appointment for the Sky Garden or consider having lunch up there.

my husband is a big James Bond fan and the London Film Museum has an
exhibition.

We did this. Not sure it would meet your extraordinary requirement, but it is a fun couple of hours for Bond Fans. What I considered extraordinary was Tower of London, Hampton Court Palace and Warwick Castle.

I can second a trip to Windsor. It's our favorite castle! And our kids really enjoyed it , too. I've heard, but never tried it, that you can go 'mud larking' on the banks of the Thames. You walk along the side of the river when the tide is low and sometimes you can find interesting 'antiques'- sea glass, old pottery shards, knife blades and other iron works, etc. With that many days, I'd plan on day trips about every three or four days. Windsor, Stratford-upon-Avon, Canterbury, Dover, Bath, Winchester, York, and many others are possible. All are doable by train on your own. I also agree on the theatre (and concerts).

You are all amazing! Thank you for your help and time! Hoping others are getting good info too!

Catch a match.

London is endlessly fascinating. Over the last three summers I've spent 10 days, 12 days and 14 days in London.

I will second the recommendation for London walks. I'm sure there will be themed walks that would interest your teenagers. I can personally vouch for that company's Jack the Ripper walk. You can check to see which walks will be offered during your trip right now and keep a list of possibilities at hand during your trip. I like to make last-minute decisions based on weather. Around the end of April you'll be able to download the full schedule with descriptions of all the walks.

About the mudlarking: I know that at least some of the time it requires a permit of some sort, at least along the Thames. I don't know whether that's always true or applies only to certain areas, but you should definitely Google so you know whether there any restrictions. At one point I read about a group mudlarking walk in London.

Yes for mudlarking on the Thames you need to get a permit from the Port of London Authority which doesn't come cheap: £40 for one day or £85 for one year. This only allows digging to a depth of 3 inches: for anything else you have to be a member of the Society of Thames Mudlarks.