So I just did a quick google on London and I don't know if I'm missing something but it seems like senior software developer salary is less than the avarage salary (senior developer 77,500, avarage is 86,400). On glass door I see 50-70k GBP for senior developers.
This sounds horrible, but I have a feeling London/Wester Europe is unique in this regard, I can make that kind of money remoting from Croatia.