Summer temperatures average highs around 70 degrees and lows around 50 degrees. Winter temperatures average highs around 40 and lows around 30. Everything in London is expensive compared to Southern California. In most cases you'll pay twice what you'd expect for everyday items. Eating out is particularly expensive.
Hotels are similarly costly.
If you're interested in enjoying London's night life, you need to be prepared. The one area where you'll find that London is cheaper than the U.S. is live theater. Tickets for a show on the West End are often half of what you'd pay on Broadway. Since many of the great Broadway shows started in London, this is one bargain you can't afford to miss. To save money on tickets ask for obstructed view or student tickets, which usually mean you'll sit in front of a guard rail.