Same boat as you, and with this damn ULEZ coming in April 2019, I'm forced to swap for something else as I commute daily into centre. But I don't really have space/funds for commuter bike + weekend fun bike.
Like you, I like the balance of power, economy, comfort and practicality. Other things that put me off replacements is the lack of tank range - I'm gonna really miss getting 230 miles between fills.
I've looked at:
FZ6 S2 - obvious option. Seem to be fetching £2.5-3k odd recently.
CBF600s - felt a bit more torquey, but less top end power. Also very bland looking, but are cheap
SV650 k7 - possibly an option, but 73 bhp on paper and range of only 160 miles.
ER6 - meh, basic learner bike, too weedy and budget.
Gladius - ditto, even if just a cheap updated SV650.
CBR600F - nice option, but insurance is silly for inner London postcode (£1.2k odd!)
GSX650F - looks alright, but bit on heavy side, 150 mile range, basically a bandit in nice fairing.
BMW F800S - Seems a nice practical commuter, but reports are it can be rather tame/bland to ride (especially compared to FZS).
MT07 - has the looks, but looks a bit whimpy on power compared to FZS600 and bit dinky for tall rider.
MT09 - looks fun, but too pricy still - so insurance is too high in London.
Tracer - too wide for London computing... Seen some poor reports on longevity (corroded leaking oil pump on front of engine after 2 years!)
Street Triple - Silly insurance in London.
So yeah, I'm still no closer to a decision and time is ticking on...