| I just got to the UK yesterday and hopped into my hired car (no, not a MINI). I found the transition pretty easy, I haven't driven over here in 15 years or so (and even then, not extensively).....drove around Maidenhead, Wokingham, & Windsor and then down to Stonehenge (about an hour southwest). I find the roadways well marked, and the roundabouts well designed.
I think the biggest drawback to a RHD in the US or vice versa would be trying to get in/out of gated things, in addition to toll booths as was mentioned previously. Tho' w/ the MINI it's fairly easy to lean over ;-) | 
