Haven’t been to Paris for years, so can’t comment, but know London well and it’s a great city to visit with a huge variety of shops, restaurants, galleries and museums and different areas which each have their own character.
London feels safe, just be aware of who is around you. The tube network is easy to navigate and the best fun to be had is sitting at the front top deck of a double decker bus or taking a Thames Clipper along the river to see the whole city and you can travel from Putney up to the O2 and beyond. Portobello and Camden markets are packed but fun for late teens.
Avoid mid-Oxford Street as a crowded tourist trap nightmare (Selfridges fun for teens though) but the area behind St Christopher’s place and streets behind this area up to Mayfair are great. Covent Garden and Soho with Chinatown and the theatre district are worth a look, the public parks are wonderful and London Bridge or Blackfriars are the stepping off point for Borough Market and a riverside walk to the Tate Modern and all the buzz of the South Bank. Chelsea from Sloane Square down King’s Road is also great.
Two things to be aware of: I would recommend not wandering around with mobile phone in hand trying to navigate around town, as there is a risk of phone-jacking in tourist areas, often by thieves on bicycles. Tuck phones away, get a wee paper tube map, a pre-paid travel or Oyster card and tap in and out of the tube and on buses rather than using the phone. Or get an Uber or lovely black cab.
Secondly be aware of the very very small risk of spiked drinks and stick together, this would apply anywhere in the world I think.
Hope they have a blast!