I played an eub for a couple of years when a decent double bass was not practical or affordable, and personally, I kind of wish I hadn't started that way! I found that, while it got me used to the longer scale, I hadn't learned how to get a decent tone out of an acoustic without a lot of effort, which was a bit of a stumbling block when I first moved over to the double bass. In my opinion (feel free to disagree!), the EUB is either a cool instrument in its own right (think Eberhard Weber) or a travelling/loud gig tool for double bassists, not a great transitional step to learning double bass.
Re. cars, I've had my bass in cars as small as a Nissan Micra, with the front seat reclined, neck heel on the front passenger seat and the scroll in the footwell. In a larger car putting the back seats down will work. You'll get it in most things as long as you don't have more than one passenger.
Can't help with finding a teacher in London, but I think most will be willing to help you find an instrument, and might know people with basses to sell too.