When I lived in London (before I got brave/foolish and rode a bike all year round) - I combined a 2nd hand electric scooter and the tube. Scooter was £20 - and in no way would have driven me across London - got a hour tops out of the battery and slowed significantly after 45 mins.
Shooting along to a station and then letting the tube get me the 13 miles across London and then jumping back on the scooter was great.
Downsides:
This wasn't long after the tube bombing and I was stopped a few times to discuss the scooter - was it petrol powered? Was it safe to take on the tube)
It was heavy (this was 2004 and I know they have come a long way)
It wasn't fun in the rain
Stowing it on a train or bus was a hassle (again this was a 2004 model - they likely fold better now)
Had a vague unease that I'd get stopped for either riding on the road or the pavement - potential risks of getting points on your licence.
-
In the end I actually got a Razor push scooter and got nearly the same benefit, but with no charging.
But finally I got a job with room to store bikes, and a shower and never looked back. Far faster than any other option for me, and paid for itself very swiftly (started on a £150 bike, and worked way up to a nicer one in time with the savings I'd made.)