The UK holds a special place in my heart as the only foreign country I have ever lived in. I studied abroad in London for five months and it is definitely my favorite city in the world. As a result this page will pretty much be completely biased towards London but there are many other beautiful places to visit in the UK!
Tate Modern - my favorite museum in the world. There are so many unique permanent installations and fantastic temporary exhibitions as well. Best of all, it's free! A close runner up would be the National Gallery which you should definitely also visit.
Covent Garden - at this point it is obviously geared towards tourists but it is still just such an incredible area that I would be lying if it wasn't on this list
The Markets - Borough Market, Camden Lock Market, Old Spitalfields Market, Portobello Road Market, Greenwich Market, and plenty more. These are just a few of my favorites!
The official currency is the pound. You're not going to get a great exchange rate in the UK but it is a bit better since Brexit.
If you don't take the Tube you aren't doing it right. I also highly recommend that you utalize London busses. London is also very walkable. Throughout the country trains are easy (though kind of expensive) and I definitely don't recommend trying to drive on the wrong side of the road (it is a terrifying experience, trust me).
You can eat typical British food (ew) but London offers literally every type of food you can imagine. Two of my favorite spots are the Mac Factory and Flat Iron Steaks.
PLUGS / ADAPTERS
The UK uses the annoying large British plugs that you will also find used in former UK territories around the world.
None for U.S. citizens.
I have never felt unsafe in London or the UK in general throughout the many months I have spent there. Basically just take precautions that you would in any other major city.
SHOULD YOU VISIT?
Yes! Whether it's your first trip to Europe or your 500th you will always find something new to do in London. It's a nice introduction to Europe (I know they're leaving the EU but still) as they still speak English and the culture is pretty similar to that of the US. It is an amazing place that I love so much and you should definitely visit!
I flew/am stilly flying into and out of London more times than I could easily list, so here's a quick airport guide instead.
London Heathrow - nicest of London's airports and the only one you can take the Tube to/from. However, it's only common that you'll fly here on expensive international flights (i.e. American Airlines). I have never flown out of Heathrow on a low cost carrier (LCC).
London Stansted - the bane of my existence. It is the most stressful airport in the world in my opinion. The signs are always wrong (boarding means nope not yet and final call means oh finally boarding 90% of the time). You also have to take the Stansted Express (or National Express bus if you can get a cheap ticket) which is expensive. A hub for Ryanair and other LCCs.
London Gatwick - a middle of the road airport. Low cost carriers and nicer airlines both fly here. The Gatwick Express is painfully expensive and the airport can get crowded but it is definitely better than Stansted.
London Luton - I've only flown through Luton a few times and basically it's just there. Harder to get to as you must take a bus.
London City Airport - I've amazingly never flown here so I don't think you need to worry about it. It's the most central airport though.