Sticking to major restaurant chains would be your safest bet, and definitely taking their epipen if they have one! There are a lot of great things to do in London, for an autistic teen I would definitely recommend The Natural History museum, the Science Museum, going on one of the red, open-roof London double-decker buses... London Eye, SeaLife London... remember to book these in advance so that you are not waiting in long queues, though if you have a diagnosis letter and their ID with you and take it to the front desk of these attractions, they are usually very accommodating and may let you skip the queues.
If you are going to take the London Underground, pre-order your Oyster Card and have it delivered to your address beforehand, with money already on the card. Download the TFL App so that you have a map on your phone, as the internet connection in the underground is basically non existent - but some internet service providers actually provide internet down there, so check who supplies your mobile data / home broadband and see if they have hotspots you can connect to on the Underground. Make sure you know your login and password beforehand! You could have your teen wear a Sunflower lanyard when on the Underground so that people are more aware to be accommodating and to leave space.
London is overall an extremely loud and sensory stimulating place, so wearing noise cancelling headphones and sunglasses might help your teen if they have sensitivities with sensory stimulation. There are many beautiful and quiet parks too. A lot of attractions and shopping centres in London have quiet rooms for people with autism, so enquire about these if your teen needs some space to breathe.
Of course, autism is experienced differently depending on the individual, so if your teen has any specific needs I'd be happy to suggest things that are directly relevant to them :)