I have a similar commute, with similar expectations and requests of my bike. It is a whole lot drier here than London, but my route is urban and city traffic. I am used to riding road bikes, so I didn't want to drop all the way down to a beater bike for my commute. I wanted something that was a lot more nimble in traffic, more stable at slow speed and when maneuvering, but also light, fast and performance-oriented just the same.
I looked hard at the Sirrus Pro (with discs) and the Sirrus LTD but ultimately went with a Trek 7700 FX. It ended up being a bit lighter and faster bike, with better components, and at a much better price. Opinions vary of course, but I think that it came stock with much better drive train components, wheels and tires. I did end up deciding (in my mind having) to upgrade the brakes though (to Single Digit Ultimates) so even though discs were not needed around here for the wet, I could have gotten a lot of benefit from them in terms of responsiveness and performance just the same. The 7700 FX was the best fit for me in terms of price and performance given my expectation and need. I was disappointed by what Specialized was wanting price-wise for the LTD. It still was not as good in a couple of key areas, like the wheels and drive train, but was a whole lot more expensive even so than the high-end Trek.
I have switched out the stem for something carbon and with less rise. I also switched to a carbon low rise bar too. Both changes were mostly because I had the parts, but they did save weight, ended up cutting a lot of the road vibration and chatter and put me in a more aero riding position.
Otherwise, going with what is basically a flat bar road bike has worked really well for commuting. The more upright riding position really helps with visibility and communication with the drivers. I can get plenty of acceleration and road speed which definitely makes sharing the road easier in traffic...meaning, I don't get passed a whole lot unless I want to which makes where I ride a whole lot safer. The flat bar gives me a psychotically better feel and capability at low speed and for maneuvering. This above just about anything else has been the best benefit that I have seen on the road. I ride with a whole lot more confidence and am much more nimble in close quarters with the flatbar. It's weird too, but I get a heck of a lot more cooperation and respect from the drivers on the road riding a flatbar than I do when I am riding my road bike. I can't explain it but it is definitely a nice and unexpected benefit. If I had to guess as to why, I would say that it is because of the upright riding position making it easier to see me on the road and making it easier to communicate at stops, lights, roundabouts, etc.
You can't really go wrong with a Sirrus. It is a very nice bike. Just be sure that you understand the components that you are getting and what the trade-offs will be. That's no different than what you have to do with any bike though. I have gotten a couple of Specialized bikes where the first thing that I ended up having to do was to replace the slow and heavy wheels and tires, even though I paid a good deal for them in the stock package. This time, I wanted to look at something that wouldn't require that change. Trek for some reason has figured out how to do a better job of this with their Bontrager line, maybe because it is already an in-house product that can be more easily and cheaply integrated into their line.
Good luck, and hope this helps.