My wife and I lived in London in 1988. We had student work visas and traveled throughout Europe. We've been back dozens of times over the years. London is a great place to be as a student. As mentioned above, there are plenty of things to do and the amount of young students and travelers make this a great place for young people to congregate, meet other travelers, get tips and warnings and get that international feel. We considered international travel a must for our children, so when they were college age, they traveled throughout Europe and South America. Even though we explored most of Europe and parts of Scandinavia, London was always home base because it's a mental break having everything in English!
I see you're from Penn. If your girls are knowledgeable and comfortable getting around in Philly or Pitt, or big cities in general, then London shouldn't be any different than the rules they employ here at home. Basic street smarts/people skills are universal.
Simple things like knowing how to deal with the homeless, different races of people, cops, impatient shop owners, nasty drivers, heavy traffic, Public transportation, scam artists, pickpockets...you know, common city things... these are important skills. London and most cities in Europe are much easier/friendly places than the average large US city. Even in Paris.. if you're used to city life, you will love Paris and her people. I've found that those of my friends who didn't like Paris were those who don't know much about city life.
Crime in London is much lower than in Philly or Pittsburgh...unless you're on a train after a "football match" with the drunken hordes and you're wearing the wrong soccer team jersey.
The food isn't as horrid as it used to be. It seems like they've upped their food game tremendously. I've actually had a few decent meals in London recently. I think with all the globalization, some of the bigger/better chain restaurants may have forced these places to improve their dishes. maybe a few French restaurants, US and Asian restaurants have opened enough branches to create quality competition. The downside, is now there's a Mickey D's or BK on every high street.
Blend- in Also, what we've found is that if you're white, then the less American you look, the better. Thus you avoid any animosity you may get from people assuming you're part of the loud/rude/ugly American tourist crowd. Or the drunkards and guys looking for casual hookups like to choose American women as a sort of "hook up prize".
It's the exact opposite if you're Black. Then, you WANT to look like a Black American. That way, they won't assume you're one of their minorities and treat you differently. These are general in nature but my son found them to still be true in 2016. They love black Americans because of the many celebrities but have the usual disdain for the black immigrants.
If they like theater, then the west end is the best place for musicals etc, and often rivals or eclipses Broadway for great musical numbers. Tell them to use their student IDs and here's a good trick, if they're flexible, is to walk up to a theater 5 mins before curtains and ask for seats. Usually for very popular musicals, they hold back some choice seats just in case some celebrity pops up. I've got some decent yet discounted primo seats like that.
Nightclubs...avoid the gigantic mega clubs on Leicester square, they're expensive and touristy.Unless that's what they want to se. Because also, they're huge and very tech heavy with the coolest/latest in lazer lights and sounds. TIME OUT is still a great weekly periodical to see what's popping in the club life. Often world class DJs have sets in some very cool exclusive places. Tell them to dress to kill and deploy their American accents so they will have a better chance of getting let in by the finicky door people.
Hostels are a good place in th UK. My two favorites in London were the International Student's house near Regents park and also in the same area is Regent's college which has dorms for rent in the summer. Actually most college dorms in London would love to rent you a dorm room with breakfast for decent price.
Hope and of this helps and I hope they have a great time!!