There are pseudo Walgreens, however they are called Tesco. They have hair products, food, etc. There are two located near the flats where the students are often located. One is at 40 Bernard St, Bloomsbury, and one is across the street from the Russell Square Tube Station.
Sainsbury's is one of the relatively cheaper places to buy necessities such as groceries, toiletries, and other household items. They mostly carry food, but pretty much anything you would need can be found there. If you want to save money on food expenses, the grocery store is really the way to go. And they have a bigger selection on food than Tesco. I also compared prices between Sainsbury's and Tesco, and Sainsbury's was definately cheaper. It maybe a two blocks or so farther than Tesco, but it is worth the extra walk. They have pizzas as cheap as £1. If you eat out in London you will certainly pay for it. Along with usual grocery items, they also offer a wide array of ready made meals ideal if you do not have a lot of time to make something.
I thought not bringing soap and shampoo would save space on packing but it is really expensive. Also, if you can't find something, just ask because chances are they have something similar but not exactly what you're looking for.
Nearest: North of Great Russell on Tottenham. There is one near the Holborn Tube Station.
In general, I would say that Sainsbury's has a better selection and more affordable prices than Tesco. However, if you are looking for convenience shopping, sometimes Tesco is perfect.
As far as calling cards go, I would recommend buying your card here. A lot of people who bought their cards at home weren't able to get them here. If you do buy them at home, check and see if there is a phone number to call before you go that will make it work in the UK. It's easy to find calling cards here. I got mine at a random tiny convenience store, but I had to ask for it. They had it behind the counter. Also, beware, the man told me that my card would have 360 minutes, and it had 20. Cost 5 pounds. So beware.
They say they are 400 minutes or so but its like 5 minutes for every 1 international minute used. Wasn't a big deal but just know what you're getting.
I got my calling card at a Bureau De Change and it was 20 pounds but had about 500 minutes so I would recommend getting it at one of those!
To be more specific, I would recommend what is called the Amer1stca (I usually just ask for the America First Card). It costs £5 and will get you 120 minutes of talk-time. The card has a dark gold cover, with its logo in bold across the middle. You can get it at the shop right across the street from the British Museum.
A great card to get in London is Eurocity for western Europe, USA, and Canada. It cost only £5 and I got over 400 minutes. The card itself is blue and is from Granam Telecom.
Has cold medicine of all sorts! I bought two different cold medicines and it was only £8! There is a boots located right across the street from holbon underground station. It is right next to sainsbury's at high holborn st. and south hampton. They carry all your ensential healthcare items and is similar to a mini Osco or Walgreens.
This is probably the closest convenience store located to our flats on Bedford. It is located 1 block behind our buildling, on Southampton Row next to Pizza Hut. It's open 24 hours and has reasonably priced goods. You can find products here similar to what a gas station may have.
The Holborn Pharmacy had excellent service. I came down with a really bad sore throat, and the pharmasist assisted me and I had medicine within minutes. It was located directly behind the Bedford Place Flats. It also sells hair products (a good place to purchase a UK blowdryer), make up, and other things that you may need.