Ok...for a North American, not living in London, here is my two cents (pence!)...
The 335i is a hell of a car on B roads, but LHD is a pain in in the ass to overtake anyone. If you are in trouble when you pull out...the torque is nice though!
As for gas (petrol!), as someone already stated, expect on close to 100 pounds for a tank (assuming you use Shell V-Power)..so think around $160 USD for a tank of gas. Not cheap!
Finally, if you have a later model 335i with Adaptive Xenon's, you should be fine with the MOT.
Afer being here for a year, I wouldn't give up my 335i for anything, but if I was staying here and not taking the car back to Canada, I would get a 330/335d, get better fuel economy, pay less road tax, and still have plenty of smiles when putting the right foot down.
Perhaps another option would be getting a 2008 M3 for a year or two as a toy??? Just sayin