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True that. Power corrupts, etc. But being fired out of a cannon, monster grunt is one of the great joys of two wheels. Basically you've got to think of motorbikes as less versatile bicycles. Which means you absolutely need more of them.
Fortuitously adding more to the insurance is usually pretty cheap. I'd have about 10 if I could get away with it, nothing desperately expensive.
Lightweight Sportsbike
Superbike
Italian Superbike
Lightweight dirtbike (x2)
Adventure bike
Supermoto
Mellow road bike
Pre-war Brit Single
Moto Guzzi. Pref old...That's 10! And I forgot race bike.
I may need therapy.
Jung-
Definitely a mistake to try and buy one bike that does it all. At best you end up with one capable bike that is compromised in every department.
Downside to owning lots of them is the constant maintenance and never getting any of them in top condition because another one is crying out for money/attention.
You've got to start somewhere though. At the moment, threat of theft aside, the 300 class looks interesting. Especially if you're just starting out in London. It won't be much fun 2 up out of town though.
Scooter for a first time rider, definitely consider something with ABS.
Airhead
The only downside to larger bikes is weight and running costs. The 600 class is easy to get into because the weight doesn't really affect them but most people end up wanting something larger because they're a bit more relaxed when you're riding longer distances/motorway speeds.
Personally I enjoy thrashing a small bike more than easing the power out of a big bike. I do think it's easier to get it wrong when there's more power available but that's just my slightly irresponsible attitude.