You need adaquate standover clearance. On a traditional road bike (with a horizontal top tube) this is is 1-3". For a more modern style or an MTB with a sloping top tube, clearance is designed to be 3-6".
Note that the length of the bike is just as important. If you get a sloping/compact style frame with 1" standover, then it will be too big and long.
If you don't know what you need, then you need to see the bike and try it out before buying. Suggest you ask at a more local forum such as cyclingplus.co.uk for a source of used bikes in London.
Make sure you get an appropriate style as well as size of bike for commuting.