Monday 3 August 2015 10.14am
I recently got the Gatwick Express. I had a local company take me to Victoria then caught the train. Was much cheaper to.
I guess it might depend on where you live in SE1, but there are cheaper trains out of Blackfriars and/or London Bridge to Gatwick. They may not be 'Express' but often only 1 or 2 stops, so not much different in terms of time. So when you factor in the time (and money) to get to Victoria, could be a better option.
If you have a smartphone, the CityMapper app does a decent job of laying out some options for you.