My niece did this sort of trip a few years ago, I remember a few issues.
She won't be able to get Sterling coins in the US when changing money at a bank; none of the banks here will exchange coins. The bureau d' change in the ATL int'l terminal may have coins, but she will pay a premium fee to exchange there. OP, please note that the UK has done away with £1 notes; £1 is a coin. The smallest denomination that you will be able to get at a US bank is £5, but do try to keep the denominations small. Suburban bank branches in the US often will not have foreign currency on hand and will need to order it from their HQ office; if you want it quickly, drive to your main office. (Also, don't worry about the post that mentioned not being able to get Chinese currency. Sterling is different; if the bank changes money at all they will have Sterling and Euros on hand.) I would have her take enough Sterling to at least get her through her first full day.
As to the prepaid Visa, yes, you can buy it in the US for use in the UK, but realise that because it is issued in Dollars, there will most likely be a currency conversion fee attached to each transaction, and it could be as much as about $4, depending on what bank issues the card. Be really careful using this option and be aware of the fees; it might be better for her to just plan to use the ATM every 4 days or so to get cash, because she is fairly unlikely to be purchasing any big-ticket items I would think.
Have her leave the hair appliances home; she's likely to forget the settings and fry them and they will be heavy in her bags. (There is a good chance that those dorms won't have elevators; she needs to be prepared to CARRY her checked bag up stairs.) Tell her to plan to braid/band her hair most of the time, and to stop into Boots on her first day and buy a UK hair dryer to share with her roommate; a basic one will cost about £8. That and a round brush will straighten her hair sufficiently for 2 weeks; an iron is overkill.
I imagine that her friends are going to gravitate toward eating in pubs, especially at first, simply because of the novelty of being able to go into a bar at their ages (you cannot order alcohol in a pub until age 18, but it's legal for kids at 14 to enter the premises.) Food in general is more expensive in the UK, and especially so in London; to be on the safe side, have her budget $20 for each "on her own" street meal she plans to buy. If she has money left over so much the better.
Iced tea is unusual and often unheard of except in a bottle; remind her that if she orders tea it will most likely be served hot, and if she asks for ice she'll most likely only get a cube or two. (Note that in most restaurants if you order water, they will usually still bring bottled unless you specify tap water, and in tourist areas they are fairly likely to open the bottle before serving it, thus preventing you from refusing it.)
A pocket compass is a very handy thing to have, along with shoes that can handle getting wet, especially in Scotland. Oh, and a couple of washcloths; those are considered a personal item in the UK, and lodgings normally don't provide them. Send ones that are brightly colored so that she doesn't forget and leave them in the bathroom.