Obviously many plans are limited to UK citizens who want a contract. This clearly doesn't work for visitors. If you don't mind jailbreaking and unlocking your iPhone, there are tons of good plans where you buy a pay-as-you-go sim card for the iPhone. Data rates are very cheap compared to the US. The main carriers that offer this are O2, Orange, Vodaphone, and Three.
A second option is wifi. London is fairly well blanketed, and I see BTOpenzone everywhere. TheCloud is also rumored to be good. Finding free wifi is not as easy.
A final option, and the one I went with, is really great if you ever plan to visit again in the near future, or need lots of data. The UK mobile company Three sells a py-as-you-go mifi for 72 pounds, and that includes VAT and the first 3GB of data. Top ups are very cheap compared to US data plans. The mifi ensures that you have wifi based on the 3G networks in your pocket wherever you go. I consider this the best option for anyone who doesn't want to jailbreak their phone.
Obviously since I only had a mifi, I had no access to voice or texting. A very cool trick to be able to text your friends and family in the US is this: setup a free gmail and and Google Voice account, and configure the Voice account to forward texts to gmail. Have people text that Google Voice number and it will go to your gmail account on your iPhone, where you can simply reply and it will appear as though you texted them back from your Google Voice number!
Text someone from your email
All the major US carriers allow you to email a text directly to someone. Suppose the number you want to text is 212-555-1111. Depending on the carrier, send email to the following addresses to text that person:
- AT&T - email@example.com
- Verizon - firstname.lastname@example.org
- Tmobile - email@example.com
- Sprint - firstname.lastname@example.org
If you're staying any length of time at all, it's well worth it to buy an Oyster card for the tube and the buses. A 7-day unlimited card cost me about 28 pounds, or you can buy a pay-as-you-go card and just stick money on it (they make sure to never charge you more than the max daily rate, which is nice).
There is an app called Tube Deluxe that is very useful in getting around, and the iPhone Maps application is priceless for walking and finding tub stations.
Time Out London is a nice free app for finding things to do, including a feature for finding things near where you currently are.
The free GateGuru app is great for navigating airports.