do you mean the east coast? LA is on the west coast.
LA's about 5-10 degrees warmer than the UK all year - nice and warm in winter, sometimes uncomfortable (can be over 100) in summer. It's not specially humid (because it's on the ocean) but can be smoggy. There's a decent metro system but it's a huge city and there are times you might want a car, depending whereabouts you want to go.
All sort of airlines fly there, and the fares will vary a bit according to the season. Try putting in dates and checking them on skyscanner.co.uk
This might help as a general guide
http://www.discoverlosangeles.com/