About £70k is what I see for senior devs (I'm fine with your definition) at well paying startups, but you can definitely get more in the city or the wharf at one of the financial institutions. For startups though, my experience says £70k is about what well paying startups are willing to dish out. There are outliers though, obviously. (Case in point: I know at least one startup that pay some of their engineers up and potentially even beyond the £100k mark.)

I don't know what Facebook, Amazon, Google etc. pay, but from what I've heard I'm guessing it's about in the same £70k-£100k bracket as the financial firms.