I'm a long-time motorbike rider, I've had a few 125cc bikes in the past and currently ride a Honda CBR. A 125cc would actually be so much cheaper than one of these "Stealth Bomber" bikes or something similar.
Annual parking is £100 in Westminster but is free pretty much everywhere else. Running costs of a 125cc should be no more than around £500-£800 per year with everything included (insurance, tax, MOT, parking, petrol). It would take 10 years before you even approached the cost of one of these electric bikes. Even if your 125 was damaged beyond repair you could replace it for about £1000. There are actually running costs for an electric bike too, including charging, deterioration of the motor / battery, servicing, etc, as others here have pointed out.
If the price came down I'd definitely buy one, they look like loads of fun, but the £10,000 price tag renders it a toy for someone with a lot of money to spare.