Jo, there's information on the Lufthansa Two-for-One program that is done in conjunction with BMW's ED program. You can find out more info
here at the BMWUSA website.
When I did my ED, the cost savings in using the that program was not as great as purchasing two discounted tix with another airline. So, be sure to shop around.
You might want to do an open-jaw flight, and fly into Munich, pick up the car, and drive it to the UK, use it while there and drop it off at the dropoff point at Bournside Services in London. You'll be able to have a fantastic car and save on car rental fees...