I live 25 miles from London and detest it with a passion.Konrad, Welcome to the site!
I tend to agree with some of the beginner comments, however, describe "beginner." Have you ever rode a motorcycle before? Are you familiar with the controls but never rode on the street before? First bike ever?
I have never been to London (bucket list location) and can't discuss the roads or styles of drivers/motorcyclists, but depending on the above answers, I think it may be or is doable.
While i agree with most comments here, in that it's not a beginners bike, my take would be its a very good choice if you're getting a fast bike for the first time.
if this is your first ever bike? then noooooo it's a terrible idea. In the uk you may be over 24 so in theory you could get your license and buy it the next day.
you are going to drop it and it's going to break your heart.
However, if you've spent the last 2 years on your A1 license and you're looking to move up from the cb250 for a 'sporty' bike i think the duke790 is not a bad choice compared to its rivals.
From my personal experience:
i've spent the last 9 years on a Suzuki v-strom 650. It is a wonderful bike, but not exactly 'performance'. I wanted something sporty but not racey, i needed something that i could sit comfortably on and commute though heavy traffic but also blast about when i wanted to.
So i got a duke and it is absolutely faster than my current skill level. but... the electronics save me!
With the v-strom i would have to be very aggressive to get it move about the place and when i tried that on the duke out of habit...the traction control cut in. if i dumped the clutch a bit fast from the lights...the anti wheelie cut in. Wet manhole cover under braking on a bend? active abs!
The duke makes me appear a much better rider than i am.
I was also considering the Triumph triple, and that bike would have scratches on both sides by now.
so in summary:
If you are going to buy a sports bike and you haven't had a sports bike before, then the duke790 is a good choice.
If this is your very first bike... then i recommend the v-strom, though it may be a bit wide for london.
(for reference, my bikes were cb250, then a bike i genuinely can't remember and now can't find on the internet i thought it was a CB300r but it's too new, a ZZR1100 (great bike!) a Suzuki DL650 v-strom and now the Duke790.)