When walking the streets of London (and any other major city, really), it is not recommended to use your mobile phone publicly. I have heard from several people now that they had their mobiles ripped out of their hands while talking on it, walking down a street or waiting at a bus stop.
If you plan to spend more than a couple of days in London, or if you think you are going to come here regularly (be it once a year), it is worth getting an "Oyster Card". To get this blue card, which looks a bit like a credit card, you have to pay 3 pounds at any of the major tube stations, to obtain it. Then you can top it up with travel money (online or at machines at almost all tube stations in the centre).
With the Oyster Card you have to "touch in" when going thru the gates and "touch out" when leaving the tube area again, on little round yellow pads.
Why you should get that? It is cheaper to travel! It only costs you 3 pounds for a day traveling in London city, compared to 5pounds10pence for an off-peak day pass! (and actually prices are rising in the new year). Also, traveling by bus with an Oyster card is almost half the normal fare!
Check http://www.tfl.gov.uk/ for more info on the card and for travel information within London.