I'm not sure I have a particular 'recipe' yet. For sure, if I only use one flash, it's the 580. Generally I dislike the harsh light of flash so any combination of dialing the flash down with FEC, bounce, diffusion or off-camera is preferable IMHO. I had the STE2 before I got the 580 so used it for a while with the 420 as sole flash but I've since used it with (just) the 580 off camera. I'm aware that I could use the 580 as trigger only for the 420 with longer range than the STE2 but this remains an experiment for the future. I guess it's nice having a range of options

.
I'm not sure which way I'd go in your situation. Buying another flash would give you the possibility of 1 on / 2 off camera but you don't get the option to use the powerful 580 off camera. FWIW my experience of using two flashes has been fun but challenging - particularly in terms of balance. I'm therefore not sure that bringing a third into play would be easy to do. If you haven't already done so I'd poke your nose into the strobist website and also invest in a copy of 'Light:Science & Magic' - slightly heavy-going and textbookish in places but worth persevering with.
Your £100 budget would also buy one of
these (possibly with change to spare?). I think that placement and control of light is as important as number and power of sources. I have one & whilst silly reasons prevent it getting as much use as it should (largely based on a small sports car with a tiny boot

) I think you'd find it at least as useful as your other options under most circumstances.