I rent cars in the US each year.
Why do you particularly want one of these - generally speaking unless you want to pay an absolute fortune you are going to end up with a 328i or a C250 or something which are hardly worth paying big money for. Infact I actively avoid these two cars on rental lots as they are generally poor specification as the rental firm's believe the badge appeal is sufficient.
Your best bet is to either:
a) Rent a Luxury class car with Alamo/National. This will give you something like a BMW 328i, Cadillac ATS or similar (IMHO the Cadillac is a better car than the 3 Series and thats coming from a BMW owner
b) Rent an XCAR category vehicle with Sixt. XCAR is generally a BMW 5 Series or a Mercedes E Class.
Your cheapest and most convenient rate for Alamo/National will be through Virginholidays.co.uk I would imagine - the rate is fully inclusive of all insurance. For SIXT, use the SIXT UK website (
http://www.sixt.co.uk) which will also include all the insurance you need. Beware that SIXT has strict territorial limits, you can't take it more than one state out of Florida.
I have never found it worth paying the prices rental firms tend to want for something like a 5 Series, even though I own one myself.
Don't get ripped off just because you crave a shiny badge. The domestic cars are just as fun if not more so on a US holiday. I always aim for a Cadillac when I'm renting, the current range is absolutely excellent and it just feels more fun to have an American car in America.