Start looking now for your accommodation. At this late date I’d search via AirBnB or Booking.com.
Right in the middle of the tourist season in one of the most popular Cities in the world. Make certain you stay within in travel Zones 1 & 2 or you travel into and out of the city center will be costly. http://content.tfl.gov.uk/large-print-tube-map.pdf
Teens in London.
It will be interesting to them because it’s different.
You can take them into pubs for lunch.
Riding the tube will be new.
The sights, sounds and smells will be different. The crowds will open their eyes. The accents will open their ears. Full English breakfast with tomatoes and beans will be different. Bangers and mash. Candy is known as sweets and will be different although they can find a Snickers bar.
Language will be different:
Lift is an elevator
Crisps are potato chips
Chips are large fries
Loo is the restroom
Nappy is a diaper
Flat is an apartment
Trainers are sneakers
Telly is television
Biscuit is a cookie
So here’s my suggestions to keep them entertained:
Rent Notting Hill the movie or stream it...watch together... then visit Portobello Market.
Camden Lock Market then walk along Regents Canal.
Brick Lane Market. Lots of eye candy and food stalls. Do the Graffiti tour.
Go boating in Regents Park.
Walk past Kensington Palace or tour it. Explain it’s where Prince William and Kate and Prince Harry and Meghan reside. Enjoy the waterfowl at the Round Pound.
Do some London Walks. Pretty certain they’d have interest in Jack the Ripper.
Cruise the Thames.
Are they Dr Who fans?
Harry Potter Fans?
Take them to Carnaby Street.
Pop inside Liberty.
Take them to the Woman in Black.
Picnic in Hyde Park.
Lastly check TimeOut London online to see if there’s something of interest. You could always try the new zip line or try the Tower Bridge Experience. https://www.google.com/amp/s/www.standard.co.uk/go/london/attractions/the-world-s-fastest-zipline-is-returning-to-london-a3821126.html%3famp