Why the 3 days in Florida? That's 3 weeks on its own.
To answer your actual question, 14 days but depends how long you want to stay in SF, LA and SD, also how many stops in between. £5k should be fine.
Monitor flights on sky scanner via email alert, buy when it drops. Early does not equal cheap, prices vary wildly right up until boarding. Though fling to SD or SF is more expensive than LA so I am note sure what you should be aiming for there and its also complicated by you not doing a direct return.
Yeah would probably bin off the seperate trip to florida and do that as a holiday in itself. Thanks for tips I'll considering setting up skyscanner notifications