This doesn't have anything to do with London, just the rarity of Rocksters.
There is a stretch of highway near where I live called the Dragon (Hwy 129 ending in Deal's Gap near Robinsville, NC). It's got 308 turns in 11 miles and bikers from all over come to "play." The is a rider/photographer called "Killboy" who has a staff that photograph every vehicle that comes down the road and posts it on the web to sell the photos. They post all photos in the store but feature their favs on the front page of their website. Their favs are usually funny shots, hardcore sports rider with perfect form, or just cool looking bikes.
That's a long set-up just to say: every time a Rockster goes through there it winds up on the front page (there's one there for this week:
http://www.killboy.com) with a comment about the design or look of the bike.
In my personal experience, I've never seen another one on the roads, but there is one somewhere around here. People keep asking me where I was going at some certain time of day, or why didn't I wave, or when did I get a new helmet, etc.