i'll have to agree with some of the others, september isn't really a good time to visit florida. way too hot and hurricane season is at it's peak (we got whacked twice in three weeks around these parts in september of 2004), so not a good time to be there. imo, november-april is probably the prime time to visit florida, rest of the year, it's way too muggy and you have a higher chance of running into storms and hurricane season. also, i think the theme parks in florida have too much of a kiddie flavor to them as well as a tourist trap center.
i would stick to the west coast in september, weather's great (santa ana winds don't come until october), lots of natural places to visit, etc. i would spend at least 3 days in san diego as well and if you are into gambling, at least two in vegas (4 hours from la).
here are some san diego attractions.
beaches (esp. for surfers)
roller coaster at mission beach
sea world
babola park w/ plenty of museums (including the san diego zoo). i would suggest you go on a tuesday, since some of the museums are free each tuesday (the ones that are free are based on a rotation)
old town san diego
scenic areas all over the city
if you go to vegas, go to lake mead/hoover dam, which is less than an hour away, and maybe transition to the grand canyon.
if you want to spend less than a week in la, you can head over to sequoia national forest or yellowstone for a couple of days.
based on that, he's a good plan.
london to la, then drive to sd, spend 4-5 days in sd
rent a car, drive from sd to las vegas/grand canyon 3-4 days
drive back to la (1 week)
drive us 101 to bay area (5-6 days)
head back to la (via national parks?), fly back to london
that's just my plan.