There is also a general acceptance of street photography, which means you are able to take shots without trouble from either your subject or from other members of the public. Much like most major cities, everyone is far too bothered about their own business to worry about you!
The most important thing to remember about London is that its famous weather is very unpredictable. One minute it can be bright sunshine, the next overcast and feeling much colder. This is not a problem for those who know this, but just means that you generally have to be prepared for all eventualities – whatever the time of year. Just bring a few layers and comfortable shoes and all should be well!